Each manufacturer sets the time til expiration for their car seats. Some manufacturers even put an expiration date on one of the labels. Check the date of manufacture, which must be on one of the seat labels by law. Most car seats expire 6 years from the date of manufacture, unless there is a different expiration date marked on the seat. If you're not sure, call the manufacturer. If the labels are missing from the car seat, it's best not to use it, as the labels would also give you important model information that would alert you to potential recalls.
If you cannot verify all of those things, the car seat is considered unsafe for use, even if it appears to be in good condition. Crashes can cause stress and structural weakness that isn't visible from the outside, and older car seats can be weakened by many seasons of heat and cold in the car. Unless a used car seat is coming from a close friend or family member and meets the above criteria, it is almost always safer to choose a new car seat for your baby.