There are some guys out there making 5k or more a month however like @bodyinbeta said even if you're making 15k a month, it could all come to a halt any day if Amazon suspends your account or ends the program or changes the commissions or base prices of shirts. Because its a third party ie you don't control it you don't have much control over your situation.

It is a nice little side hustle though, can be very passive and you could always expand it into an apparel line or tee shirt store on shopify as well or something so lots of room to grow it.

I'm a huge fan of having multiple income streams so get merch rockin, do youtube, publish some kindle books, maybe create a digital product, get a whole bunch of different things going.