They are both ok. UK English may favour using "it". "I like IT when ..." is certainly more common in my part of the world.

I like it when is probably more common, but both of those are not full sentences with full context on which to judge them. I like when is also common and used frequently, but UK speakers would instinctively place an it in the sentence for daily use. I like when would be used poetically for poems etc. .

:) Thanks

In the US I think both forms are used pretty much equally. To me the version with "it" sounds a little better, but in casual conversations I sometimes just say "I like when (the theater isn't too crowded / my mom makes cupcakes / I don't have to get up early / etc.)"