I recall, from looking for info on JUCO eligibility and what's required of them before they can enroll and play at a D-1 level, that:
1. If a player is an academic qualifier, and chooses to go the JUCO route for some reason (underrecruited, etc), they can transfer to a 4 year college anytime and play immediately, even after one semester or one year as long as they're in good academic standing.
2. If the player is a non-qualifier academically, they must complete two years at the Junior College level before they can transfer and play. In fact, didn't Sam Cassell go to Junior College for two years but only played one and thus had two years' eligibility left at UConn?
But don't hold me to this being eggsactly right.