Children with autism can become distressed if they do not know when an activity starts and when an activity ends.  Some activities have a natural start and end, for instance puzzles.  If you give a child an activity like playing with cars of a favourite toy, these activities do not have a natural end.  This explains why children can become very upset when you try and stop them from playing with a favourite toy.  To help children understand when they need to finish an activity, I use timers.  Sand timers can be very useful as the child can visually see how much time they have on an activity.  You can buy a great range of sand timer on ebay:

http://shop.ebay.co.uk/?_from=R40&_trksid=m570&_nkw=sand+timers

They usually have lots of timers for different amounts of time at good prices