On my viola I try to do two small practise sessions a day of 15 minutes each, over the last week I have been trying to build this up but I just can't play very well after 15 minutes at all! As I'm going back to uni and work this week I was wanting to do one session of 30 minutes per day rather than two at 15 minutes per day, anyone got any tips how to build this up? I don't want to be doing only 15 minutes per day as I really think my viola playing will suffer.

I am afraid but I think the only answer is 'patience'
when I first started violin I didn't concentrate so much on the time I was practicing, but rather said to myself: I will practice until it does not feel comfortable anymore or I don't sound decent anymore LOL
I gradually found that I was able to last longer and longer, until today that I 'think' I could comfortably play for at least a couple of hours before I need to stop (although most times because of time constraints I stop at 1 hour).
It will take different people different amounts of practice/time to able to go on for longer, and maybe for some people it will never be more than 15/30 minutes before they need a break. Mind you, you might find if you have a 10 minutes break, you might be able to do your other 15 minutes.
Experiment a bit, but most importantly don't carry on playing if you feel any aches or any muscles getting tired.

I think it's very unusual for anyone to practice for more than 1 hour or in a single session without taking a break, if you're concentrating well and using the time effectively. The best way to build up practice is to plan your time carefully, breaking the practice session into 5 minute chunks and knowing what you will do each 5 mins through the practice. Ideally though, doing 2 or 3 sessions of 15-20 minutes is so much better than one session of 1h. I think it would be unlikely that you'd do any physical damage to yourself practicing 30 minutes a day so if you can think about planning your practice well you should be able to jump straight in and start with it now

I think this is something that just happens with time. When I began playing the cello, practising for 15 minutes was the most I could manage without serious pain. As my technical skills improved, especially my bowing, I felt less and less discomfort and almost without realising it, my practice sessions increased to 30 minutes most days. I often work now for 45-60 minutes, but always get up, stretch and walk around once or twice in that period. Little and often can be a good way to go. I have my cello on its stand so it is easy to fit in 2 or even 3 10 minute practice sessions if the day is busy and/or I am feeling a bit pathetic! Experiement and be patient/kind to yourself - I am sure this issue will seem like a distant memory soon.

I think it's fair to say that your average beginning learner won't do much much more than one 10-15 minute session of prac each day, esp. as an adult with a busy life.

Holding a viola is not an easy thing to do and it takes quite some time to build up the 'stamina' for it. Just do it in fits and starts, (ie during a 30 minute session, play a bit, then rest a minute or so - go to the loo, have a drink, send a text etc - then play again).

Thanks all will start experimenting and see what works best!

Bohemian - I do exactely that with Clarinet (breaking down into small chunks) so no reason I can't with Viola

Elisabeth - so true on holding the Viola, when I went Viola shopping I quite wanted a 16" Viola but there was no way I could manage it, my little Viola I struggle with after a little bit of time but think I'll be OK once my stamina is built up

Excellent tip everyone thankyou