I have seen discount store tents degrade to nothing in a few weeks of strong sunshine. I have seen the nylon on the south side so tender that it was hard to wad up to throw away. On the other hand the north side was still pretty strong.
Ah yes, you tend to get what you pay for. My experience with better quality tents (REI, TNF) in extended use has been much more satisfactory - no noticeable degradation after several month's of service.