Our x-box is set up on the main TV, so ds tends to be kicked off when someone else wants it. At the moment he doesn't get that much of a look in as I've got hooked on Monster Hunter and dh/dd are playing Final Fantasy. Generally speaking I think ds plays too much really, but it's the centre of his social life, so I can see the appeal. He is 14.
threesugarsplease if you go to settings, family then options it is in there but be warned the timer doesn't reset overnight it only resets 24 hours after the full allocation has been used, does that make sense?
The timer is no good if there are more than one DC using it. Ours also on the main TV and so they have to come off when someone wants the tv. We got the xbox when DS1 was 12. DS1 now 17 only goes on occasionally and DS2 maybe just at weekends. I set no limits because they are not needed but it was different when they were younger.