You say "It's time you ___" when you're telling someone to do something that you think they should have already done a long time ago:
You know, it's time you got off your ass and looked for a job.
It's time you learned a lesson about respecting your elders.
Notice that you use the past tense of the verb after "It's time you..." In the example at top, the speaker says "got off" instead of "get off".
You can also say "It's high time you..."