Warning: the links go to medical Wikipedia pages and they show bodies that look quite ill. Follow with caution.
Just isotonic sugar water will likely lead to death by scurvy, beriberi, and kwashiorkor, while also experiencing pellagra, likely before 10 weeks
Our magician gets a diet that is pretty much akin to (or rather: worse than) the Imperial Japanese Navy during the Meiji Era. Sailors in the IJN got a diet of tea and white rice for free, with no extra vitamins or sides. Any extra like greens required the sailors to pay for, and so most did not. The result of this one-sided food was a massive vitamin B1 deficiency, also known as beriberi - and in feeding sailors a more balanced food that contained unpolished rice and other grains, Takaki Kanehiro discovered vitamins in the first place.
The effects of a lack of vitamin C from fruit were known in the West much earlier, even if not understood. However, it was known that lack of fruit would lead to scurvy. Scurvy was particularly common among British sailors till fruit rations became a thing - partially due to how James Cook managed to get a world tour without scurvy after handing out lemon and lime juice.
The lack of vitamin B3 will result in pellagra. Pellagra is well known to be the result of too much corn-based food aid to Africa. The mage's very deficient diet does not provide this vitamin either. However, pellagra has a slower progression than beriberi or scurvy, so it is unlikely to be the actual killer, but it will add to the suffering of the mage.
The massive lack of protein however will result in one of the protein–energy malnutrition illnesses. As the basic calorie intake is covered, kwashiorkor is the most likely to hit the mage. Again, this illness is known to be a result of monotonous, carb-only food aid.
In fact, pretty much any micronutrient deficiency has a chance to show some of its ugly faces before the mage expires, all of them adding their own stamp to the painful martyrdom.
It is remarkable how fast scurvy and beriberi can set in. First symptoms of scurvy on British ships tended to show up within a couple of weeks in weaker constitution sailors before fruit rations were given, and beriberi has a similar onset rate. More sturdy sailors might go about a month and a half before the symptoms show. Kwashiorkor will show itself as early as the end of week 2, but surely will be present in a quite pronounced way by week 6.
Without a more wholesome food, 2 months would be quite generous till the onset of serious symptoms, and at best I would give the mage about 10 weeks till they are unable to move from either or both illnesses and suffer from life-threatening complications. Two more weeks might be gained if the mage had a particularly good constitution before this diet.
Beriberi shows at first with fatigue, weakness, irritability, as well as tingling or numbness in the extremities. As symptoms worsen, muscle weakness and difficulty walking show as well as arrhythmic heartrate.
Scurvy shows at first with fatigue, weakness, irritability, and muscle aches. Its worsening symptoms include swollen and bleeding gums as well as joint pain.
Pellagra shows up with fatigue, weakness, and loss of appetite within a few weeks, combined with a sensitivity to sunlight. This skin condition worsens to a very red rash that is also scaling, and blistering, mimicking a non-healing sunburn. As already mentioned, compared to other vitamin deficiencies, pellagra goes on too slow to be the acute cause of death from this diet - but untreated it can result in confusion, irritability, cognitive impairment, depression, and a specific form of cognitive inability to remember called pellagrous dementia.
Kwashiorkor will show up with fatigue, weakness, and irritability as long as protein reserves last. When those stocks are out, muscles are cannibalized, leading to slimming arms and legs. Edema of the feet and belly start to form shortly after, and hair thins and starts to fall out. By the time of estimated death, the mage also is listless, irritable, and mentally apathetic.
Do note that other, more long-term deficiencies exist and were common among sailors. One example would be Iodine deficiency and its accompanying goiters. Those however only show up after months, at which point the mage already has succumbed.
There's a general rule of three in first aid: Three minutes without air, three days without water, three weeks without food. The rule of thumb is, that at those points the body starts to cannibalize itself to a point that averse symptoms begin. It's clear that any caloric intake without any other sources of vitamins and protein could extend the starvation time, but there is only so much time that such a liquid-sugar diet could be used without the effects discussed above. That time I would pin as possible before other food needs to be provided to halt the downward spiral would be about 4 to 5 weeks. Such a fasting spree would still be a hard time for people, and clearly have a heavy toll.
Maybe the most comparable might be a Carthusian monk after Lenten fasting on a diet of vegetables, dry bread, and water in less than their normal quantities - though their insistence on veggies would manage to prevent almost all of the deficiencies explored above.
Fruits & Nuts + Magic
A small amount of real food, especially fruit and protein-rich nuts, could complement an almost-pure-magic diet to prevent deficiency illnesses, or at least delay their onset by as long as they are available, and thus extend the survivability.
Captain Cook famously gave every sailor a daily ration of 30 ml (1 oz) of lime or lemon juice and had not a single case of scurvy on his voyage. The RDA for Vitamin C suggests the equivalent of about 2 full lemons, which is roughly twice as much as Cook gave, but his sailors also had a somewhat varied diet with other sources of vitamin C than lime or lemon juice. Estimating from this, I would estimate that about two fruit and a small hand full of fatty and protein nuts per day would make enough complimentary food to make a full meal and skip on transporting carb-based food. However, do note that such a diet will not feel filling in the slightest.
For a more filling alternative, vegetables or other greens to complement the magic drink might manage to not only fill the stomach, but also prevent the symptoms of deficiencies fully.