I never really got into B/W for some reason (and subsequently never even played B2/W2), but I'm considering picking up White again at least to trade the starters over to Y. I know I need to wait for Poke Transporter to do this, but I'm just curious about what kind of commitment I'm looking at before I can start sending Pokemon from White to Y.
I'd move this to Quick Questions, but the question isn't part of B/W.
You cannot trade between generations. You haven't been able to trade between generations since Gen 2. There is no active point needed to reach in the fifth Gen games in order to transfer, the Pokemon have to be in your PC though.
Deposit Pokemon into PC.
Turn on Pokebank/Pokemon Transfer
Select the PC Box with the Pokemon you want to transfer to the bank and move it into the bank.
There isn't a specific time you need to do this.
If you want to trade between Gen 5 games, you are not able to trade until you have received your first badge and the Live Caster/C-gear.