How much exercise should a young cat be getting?

I have 2 young cats one is 2 years and the other one is  14 months. How much activity should they be getting at some stage in the day? They seem to be maximum lively in the early morning after which calm down for the rest of the day for naps which I expect is ordinary for a cat.

How long does each day I need to try and get them to play aside from their chasing of each different?