it's 7 days, not 15. the point is that if you're on a trip longer than 7 days you have to print them while on your trip, which might not be easy.
For the life of me, I'm sure it is (or at least it was) 15 days. And yes, printing it while on your trip can be a major nuisance (happened to me once; it was not a nuisance for me on that trip, but depending on where you are, yes it can be).
I've flown with Ryanair plenty of times and, if you are able to ignore the constant bugging with booze/parfums/car rentals onboard and can stand the small seats, they are the probably the best thing that there is.