Your diet provides humans with enough calories and vitamins and minerals from 6 per day mangos @ 210 calories per mango, depending on their body weight and activity, to survive for many years.  But you are only providing ~1 gram of protein per day from their ration of termites, ~ 8 grams from the mangos, and 1/2 gram from the celery. The human body needs 0.8 grams of protein per kg of mass every day.  If it doesn't come from the diet, it comes from the body.  

So on average, an adult male doing nothing will consume 43 g of body mass every day. The typical male has 30 kg of muscle and 10 kg of fat.   Every hundred days they'll lose 3.3 kg or 10 kg per year.  Assuming a healthy adult male, they'll survive at most < 4 years. More likely 2-3 1/2 years.  This is assuming they are doing nothing but eat and sleeping to conserve energy.

If they are active, then they'll burn their bodies faster and die sooner. And, it assumes that they don't have any latent diseases like a dormant cold or flu.  Once the body weakens it won't be able to fight infections or diseases.  

Living in alien zoos they are likely to be in contact with many viruses and bacteria they've never encountered so their immune systems will be in overdrive, which will also shorten their life expectancy under these conditions.