A high-quality, well-made cotton futon will last a long time. DH and I have been sleeping on ours since 1999, and it's still in great shape. They do tend to get firmer with time, but if you like a firm bed like we do, that'll be fine. We turn ours regularly, and steam clean it a few times a year, and then drag it out onto the lawn on a sunny day to dry and air. (Put it on cinderblocks or something similar, while it's drying, to get airflow underneath.) I expect ours to last at least another ten years. (Ours is not innerspring. I don't think it's a true futon if it is.) A wool topper will also prolong the life of the futon, as will waterproof protection if you have small children or pets.
Cheap futons won't last long at all, and will be very uncomfortable after only a few years, because the stuffing will shift around. My old roommate and I bought one cheap when we were just out of college. I slept on it for a few months and it was shot.
I highly recommend White Lotus futons. All our mattresses are white lotus futons, and they're the best futons in the business, I think. And no, I'm not affiliated at all.

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