Canvas is great on a boat, usually waterproof, UV resistant, strong and long lasting, Sunbrella even guarantee their canvas for 10 years so long as it is looked after.
Looking after it is easy, a fresh water soapy wash, make sure that it is tied down and not flapping in the breeze, and be gentle on the zips. Depending on the stitching thread and the environment it is in, the canvas could need re stitching every now and then.
Once though it is gone, it is gone, don't try and get those extra few years out of it as you will only do more damage to whatever it is supposed to be protecting.
I have just cleaned a mainsail that has been under a canvas cover which should have been replaced years ago. Firstly it leaked so the sail was always wet, thus moldy, and secondly as the canvas broke down the colour rubbed itself all over the sail, a nice tinge of green which does not come off is the result.
The sail now also has wear marks where it was rubbing too.
Your trimmer/sailmaker will tell you if your canvas has more years left in it, or bring into us here at Vacuwash and we check it out for you.