Assuming a technological level similar to or slightly more advanced than our current one, what is the longest amount of time that a front line, foot slogging soldier could expect to survive and carry out all their necessary combat duties while wearing a level of protection equivalent to something like MOPP Level 4 protection, especially in an environment where entirely unable to expose any parts of themselves to the outside environment?
Why? Because diving suits, space suits, hazmat suits, are not designed for combat-grade activity. In comparison, they're designed for very lazy energy-conserving actions. Worse, those suits severely limit the freedom of the soldier, meaning the soldier would likely die more quickly as the enemy snipers play "pop the balloon!" than they would from the environment they're trying to protect themselves from.
But minutes is an unrealistic answer because despite the military's traditionally slow process for updating wearable equipment, it does try. What I mean by this is that unless you're asking for a specific, "the army suddenly finds itself needing MOPP Level 4 protection and it cleans out every civilian warehouse to get as many suits, etc., as possible in 48 hours" kind of situation, the reality is that the military would quickly work with manufacturers to redesign the suits for combat purposes.
In which case the answer is...
However long you want given the restrictions of carrying oxygen on your back and a thermal exhaust that would show up on scopes mounted on the moon.
Which is why one of the technological coolnesses found in the Traveller universe were (if I recall the name properly), "cold cans," which were canisters attached to the exhaust that absorbed the heat to keep the wearer of the suit (battledress if I recall) from displaying an ultra visible thermal signature.
And they created a consumable that caused logistical problems that helped make the game fun.
It depends on the temperature
The main issue with hazmat suits is they're hot as hell. They keep in heat and sweat and so if it's even mildly hot you can get heatstroke very quickly.
15-60 minutes if it's very hot to be moderately functional, longer if it's notably cooler.
You can slowly walk around for longer, but if you need to do serious physical exertion you'll burn out fast.
It depends on the type/construction
WWII type suit - heavly depends on enviroment in high temperature areas 15 minutes be max. arctic climate - hour. When temperatures are 10-15C then up to 4 hours(air filter need to be replaced)
Our tech suits - from 1 to 4 hours. They are more thinkered to prevent soldier from overheating.
Space suit - up to 8 hours, main dependance are batteries and oxygen storage.
Near future tech suit with exoskeletion, high presure air tanks and einvormental module - up to 3 days. After that soldier brain will pass out with delusions.
I'm by no means an expert on the matter and I haven't done all the math with BTUs and such, but I've been in the military for about a decade and worn MOPP 4 in a hand full of different environments. You will definitely become overheated very quickly, but the M50 gas masks we use do have drinking straws which might help delay heat related injuries.
As others have mentioned though, chem gear of any variety isn't really meant for ground warfare. Basically, if the enemy is using chemical or biological weapons it means they themselves don't plan on invading on foot for some time (they probably wouldn't gas their own soldiers after all). MOPP gear is designed to protect personnel until they can seek shelter, leave the contaminated area, or for very limited recovery operations, not for use during armed combat.
Furthermore, the ultimate designator of time is going to be the MOPP gear itself (keep in mind, I'm using MOPP gear or "JLIST" as the standard) Once the package containing the MOPP gear is opened, it's effectiveness begins to degrade quickly. After 60 days of being opened (not even actual use, just unsealed and left to sit) the gear is no longer effective. Additionally, it's very sensitive to liquids, including any chemical agents in the air, on the ground, or even your own sweat and indeed urine from within the suit.
Bottom line. If this environment is as harsh as you say, the best case scenario is 24 hours of passive activity with a fresh supply of filters. If the person is forced to run and fight, I would agree with the other responses of a few hours at most.
One of the roles I had back in my army days was an ABC (atomic biological and chemical) purifier first respondent for our base (excuse me if it's not the exact title in English, I'm translating the title from another language), basically that meant that should the worst happen and there was a bio/chem attack on our base I would be the poor (likely soon to die) schmuck that was sent in to clean it up.
Keep in mind our base was in a very hot desert so I know from first hand experience how long you can do very hard work in said protection suit (we didn't use the MOPP suits you mentioned but a similar alternative).
To answer your question it all comes down to not the individual soldier but to the unit, an individual soldier alone wouldn't last an hour before heat stroke would finish him off, the standard mode of operation is to work as a unit where 4 cleaned up the gunk while wearing an ABC suit that was covered with watery rags (seriously) and the fifth member of the unit job would be to make sure all 5 unit members rags were always wet, this meant running back an forth to the nearest water source with buckets to dump on everyone (if he was lucky there would be a firehose nearby to use), we had training lasting hours like those, but once the rags started to dry you would be calling the bucket man over in less then five minutes as those things got crazy hot when dry.
If your idea is to make an offensive in those suit you can forget it, non of your soldiers will live long enough to even see the enemy.