Agreed Shipper. Sitting in it for at least an hour prior to your first flight is a great idea.
I had some troubles with my helmet too, took a few attempts to get it right. The guy doing the helmets was really helpful though, and we got it right in the end.
I personally don't think that 15-20 mins will be useful, given each sortie is at least an hour and you will have the helmet on for a bit longer than that (going through pre-flight checks before the prop is started). 15-20 mins won't even get you in the air. You still will have about an hour to go. From my experience it took at least 30-40 mins to get the hotspots appearing.