The banners are never going to last that long, they just aren't designed to.
However, most of these banners are "seasonal" so they only hang the set of banners you are quoting for ~3 months at a time every year. Then they take them down & store them until the next year when they re-use them for the same season. Make sure you put the expected life of your banners into your quote and warranty them from manufacturing defects, but NOT installation/storage problems & give them an installation/storage guidelines sheet if you get the job. Most people like to fold banners which destroys them.
If this is anything like the quote requests I get here for pole banners, outsourcing printing them is the best way to go, but even with a minimal markup I have still been way under bid by other companies in the area.