Well, it's hard to say, we need to know what type you have. I did find this info, it's on (Tuberous Begonia), and I don't know if you have that one, but this is it for the tuberous type:
When growth reaches 2-4 inches high, transplant to pots, allowing one bulb per 6 inch pot. Use a porous, peat based soil mix, covering the tubers with not more than 1 inch of soil. After all danger of frost is past, put the pots outside or transplant the begonias into the garden, setting them 12 inches apart.

Another thing I read while researching for you,
When to Pot your Begonias
When the leaves are 1.5 to 2'' long, transplant carefully, not to harm the new brittle roots.
Hope that helps, kudzu9!
Shell

Shell-
Thanks. Unfortunately I don't know exactly what I've got since the seeds were simply labelled as "Hardy Begonia," but the info you've provided is just what I needed.

Oh that's great, I'm glad you got what you needed! :-)