For me, I give myself three days to produce a track, this includes any research or setup, composition, recording, and lastly mixing for in-game. After that I test in-game and see if myself or any of the team can think of anyway of making it work better. Initially I leave it for a while and move onto the next task, that way I can come back to it with fresher ears/mind.
Production size definitely changes the amount of time you need to complete the task, this could be anything from longer tracks to recording solo musicians. Again for me I like to work in threes for whatever reason, if the track isn't finished in those three days due to it being a larger composition/production I move onto the next thing. I seem to be more efficient that way.
I have yet to take more than two projects on at the same time, so I don't have must experience with this side of things, though I try to prioritise everything, planning is key here. What is needed first? Can I spend a week on project A and then another on project B? These are the kinds of questions you need to ask yourself. You may be thinking, well what if both projects need content produced within a short time frame and the deadline can't be pushed? Umm... FLEE!!
No really what it comes down to is accepting that you will just have to move forward even though in your mind it may not be 'perfect' yet. The best you can do is to avoid this situation as much as possible, since at the end of the day you don't want things to sound rushed. It all comes down to experience to see how you personally can make it work for you, just like a lot of things, the more you do it the faster/better you will get at it.
If you are looking for a way to shorten the time it takes you to produce a track, I recommend giving yourself the challenge of composing music every single day (length doesn't matter, every little helps
). Give yourself a time restraint to work within, this is the important part, just make sure it works within your daily routine. This could be anything from 10 minutes (one bar of music) to a couple of hours (full arrangement), it just depends on how much time and effort you want to spend on it.
Best of luck to you!