Standard advice is every ten years for normal weight people.
If the old mattress is clean offer it on craigslist- I have giant breed dogs and use mattresses as dog beds, so am always on the lookout for replacements.
If it is stained then use the new mattress delivery people- a purchase usually includes a take-away service.
Or put it out on a bulk pickup day.
In my municipality I just call the city and they will send someone any time for bulk items- it’s a shot.