Hi, can you say;
"we have only have a 45MIN set time so we have to drop a few songs"
It's okay, but I suggest "We have only 45 minutes for/to play our set."
The phrase "set time" is used to mean a specific time and I had to reread it to realize you meant "time to play a musical set" and not "a specific time."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.