How long should someone be allowed to play that is Autistic? I would say that it depends, but a good standard should be as follows:
1) Limited to the number of days with a set number of hours each day
This provides the most control over game time and I would suggest that weekdays get set to a maximum of 4-5 hours since I am assuming that time is spent for other things like school, work, etc. Also, make sure that there are bathroom breaks and some walking about in there. This means getting up and taking a bit of a stroll, you can always pause the game and come back to it. Well, usually as I know of a few that might have special saving and pausing modes. On weekends, you can allow for up to 8 hours and it should be fine, but do increase the amount of breaks and have time taken out for food.
One caveat for all this is the following: That school homework or job work gets finished before any play happens. For Parents, I would say check from time to time on the games if it happens to be one that you have concerns over. I will say it even though I assume that it would likely occur anyway.
Make sure that life is being lived and time is spent interacting with others. A good balance of this with gameplay should help out, by you should always adjust things as you feel needed. And per usual, always pre-research anything you buy before you get it. It will save you a lot of hassle.