500 would be 5000 km even at just 10 km/h which would be generous (but doable) for a Shimano chain even under roadbike conditions (where this would be roughly 200 hours). Mountain bike conditions will increase wear level a lot and they'll probably be a bit faster than 10 km/h as well. I don't know if it is really as bad 50h, but I've stopped mountainbiking when suspension forks ceased to be exotic. It also differs regionally: riding muddy in a place where the air is full of corrosive ocean spray will require far more chain replacement than riding in the Utah rock deserts.

If you ride a chain until it stops working you'll probably reach those numbers. But then you will have to replace the entire drive train which is far more expensive. The chain is the cheapest part and replacing the chain a bit earlier than absolutely necessary will greatly increase the durability of more expensive parts. As a little bonus you'll repeatedly get the joy of riding a new chain. Replacing at the right time is cheaper and more enjoyable at the same time.

No, not a typo. 50 hours for expendables and suspension service on a mountain bike (I'm assuming it's actually used as a mountain bike).

For, me that works out to 2x season. The cassettes are $300 and the chains are $40. So, the chains are treated as 100% consumable/disposable.

A better rule of thumb is miles because hours don't reflect the amount of energy put into your drive train.

Typical recommendations are every 500-750miles for a mountain bike. For the average mountain biker I think that would be more like every 125 hours at 6 mph. Mountain bike chains see much more abuse than road, and you can go further too.

You can be more scientific by using a chain ware tool to measure how much stretch there is.