Does a trampoline mat stretch over a period of time?

yes it does for HQ trampoline its about 5-10 years and for a Cheaper trampoline it would be about 3-8 years. Also it depends on how you use it like bouncing a trampoline with multiple people who are very heavy can shorten its life also if you jump on it when it has water can severely damage the Mat and the stretch out the springs.