Alot depends on how long shingles will last. #1 being quality of said shingles. #2 being attic conditions, if you have a poorly ventilated attic space and lots of heat builds up under the roof you will cook shingles pretty quick.

Snow can cause damage to, if the roof pitch is on the lower side. Water can seap back under the shingles and cause mold issues that will eat the shingles pretty good.

Bad shingles usually start curling up some and get brittle. If you see spots where it looks like the shingles are cracked or breaking, or they are curling, i would look for a new roof.

Post up some pics if you can and i will take a look at it for ya.