if you are just planning on enjoying them, you don't have to separate them at all. Just enjoy them. Yeah, they may throw a wild or cherry because of different mixed strains, but just cull- or move to a different tank- as you see them.
If you are planning on selectively breeding them, separating them would not be a bad idea.
How many generations til breeding true is a tricky question. My Nessies took years to breed one general color.
My Sapphires bred blue almost immediately, but color intensity has taken years.
My Sonics were bred from BV, so I started with a true strain and bred for darkness. That tooo took years, and I still get occasional lighter bv's thrown.
So like I said, just depends on your goal.
All depends what your goal is.
For me selectively breeding, I aim for breeding true for 3 generations.