C is now 8 months and he has been using his jumperoo from 5.5months. It is still his favourite thing in the world. He spends time in it every day sometimes up to 3-4x and never bores or tires of it. He is still on the lowest setting (but is a tall baby - and outgrown his 6-9 sleepsuits)
I think they can be used at least until a baby can walk, and C can't crawl yet so expect will get plenty more use out of it.
It has been one of the absolute best buys, I got one 2nd hand but would have paid the full £70-80 for one knowing just how much he loves it. Only caution is that i have heard some babies really don't enjoy them so may be worth letting your LO try one before you part with the cash. If the one on e*bay is reasonable price may be worth buying and reselling if he doesn't like it?