How much time would it take to learn Game Maker enough to create a basic game with it?
Depends what you mean by basic.
You can probably get a handle on everything in a few hours just by going through all the tutorials that it comes packed with.
If you want to get into scripting that depends on how much experience you have with programming. But again there are plenty of tutorials that are easily accessible.
Krakenskin Leather Works, my Etsy store.
LordVanya, my art page.
FundamentalEssence, my game development page.