When you are at a safe altitude that allows you to survive an engine failure.
Obviously depends on make & model and pilot proficiency.
1000 pilots a 1000 opinions.
I felt really comfortable reducing to climb power passing 1000' AGL.
It takes less then a min to get to that altitude and that's where I would transition to a higher speed cruise climb. Anything fails at that altitude you don't have to climb anymore, simply turn downwind.