Don't buy normal trousers belts, instead buy elastic belts which can last literally for life
(i5.walmartimages.com)
There's quite a range of quality and durability in denim, but it can be extremely durable. It was associated with physical labor in the US for a long time and especially in its old West (cowboys, railway workers, etc). You can still find a pair of jeans that will last you the rest of your life and then be sold in a vintage store, but they're not cheap. Cargo pants have a similar range but I think they're typically made of duck canvas.