As for how long you need to wait until you are ready, I'm sure it is faster, but I can't be more specific. But you may want to consider letting it ferment all the way, cold crashing, decanting the 'beer', and pitching the yeast slurry. That's the best way, IMO, to make sure the yeast has used the oxygen to build strong healthy cell walls and that you don't get any off flavors from the starter 'beer'.