"in a while" would mean after a short period of time "for a while" would mean for a short period of time Example: "In a while I can talk to you for a while, but I only have 20 minutes this morning."

Ill give an example sentence: "I haven't done this for a while now..." For a while, for me, gives feeling that the action is going to continue its existence even though it wasn't done a lot lately. I'm thinking it's a small nuance which you should not pay much attention to. In a while expresses something done after the specified time. "I'll get used to this in a while, I guess" as the other person specified Nuances are a lot in Ingrish I feel like xd