Sail care helps a sail last but the conditions we use modern sails in is a testament to their strength but inevitably can lead to fast wear.. My Tbird lasted about 20 mins in surf last week !!! (No fault of sail ;its simply not designed for use in waves but I needed a big sail to get out and paid price coming in !!!)
Everybody knocks monofilm with regards to its sometimes quick degredation but I remember ruining dacron regatta sails on almost first usage; they still looked great but dacron stretches quite easily and can do only too quickly when abused. (ie used in winds it simply wasnt designed for) I wouldn`t buy a dacron sail but I know some folk swear by them. I`d rather buy a new monofilm every 3 years than have a dacron sail 5/6 years .
Nowadays if we use sails in their intended environment;dont leave them in sun and store carefully;(not crumpled up and not standing on clew) they can last years. I`ve found Gun sails to be one of best on market for longevity or it could be that they are worth so little second hand I keep them for years.!!
I`ve found NP to be very prone to delamination after 4/5 years but I`ve got a Combat that delaminated so I pulled the thin colour film off and it still seems fine. Rigs/sails great....???
I`ve normally damaged my larger sails beyond repair before they`ve worn out !!!