Use in a while to say WHEN you will do something or when you will arrive. "I will be there in a while. I will go to work in a while". Use for a while to say for HOW LONG you have done or are planning to do something. "I was there for a while. I will be here for a while. I have worked here for a while." This is the best I can explain it, but I'm not an expert on English grammar

"for a while" EXAMPLE: How long is your class? It's for a while. When is spring? Not for a while "For a while" - This means more of what you are doing is going to take some time. "In a while" EXAMPLES: When is your class? In a while. In a while, I will need those papers. "In a while" - This means soon

Thank you so much, Jasper & Gabriela !! :)