Answers to your questions are probably in the sticky, but....
As long as it is sanitary, there is no real limit to how many generations you can repitch. You are likely to see some genetic drift after a while; either attenuation or flocculation will start to suffer.
If it's within a couple weeks, you won't need a starter at all if you have enough slurry. Up to four months or so, make a starter just to wake the yeast up. After that, approach it like you're rehabilitating your culture.
This is applicable to "normal" yeast. Weizen and Belgian yeasts are more fragile. They prefer to be top-cropped anyway.
Disclaimer: I am not a microbiologist. I don't know what I'm talking about.