I'm new to this so I am sure someone has better information but I have just made a dozen shirts using HP dark transfer paper. I waited at least and hour but I have gone as long as 2 days (actually I have one waiting for a shirt and it has been a week).
If you don't mind my asking, where did you find the HP transfers? I spent all afternoon looking today and could only find Avery and some other brands I had never heard of.