How long should I wait before hearing back from a seller if I sent a private alchemy to? 

They should respond pretty quickly, it's just like sending them a convo, so it should be within a few days, at the most!

How long has it been??

I just sent it yesterday.LOL. 

You may want to send it again if you do not hear back today.  Etsy has been having some tech problems over the last few weeks.