What are the replaceable parts?

How frequently do they need replacement?
Wear vs Usage really depends on a lot of factors. Note that the Poweriser warranty doesn't cover damages due to normal (or improper) use.

Poweriser Hooves are the most common part in need of replacement. If you are using your stilts on soft surfaces such as dirt or grass, the hooves will last years and years. If you are using them primarily to jump, they will last a while, but running and moving on pavement wears them down more quickly. In my experience, hooves last about one and a half season with occasional usage on pavement. This can be mitigated by the use of "tire socks", a tire or something rubber wrapped around the hooves.

Poweriser Springs are designed not to catastrophically fail, but over time it will wear more and more. The appropriate time to replace the spring is when you find the stilts "bottoming out" during regular usage. Again there's no established range on these because it depends greatly on the weight of the user, the kinds of usage, and any damage. My first pair of stilts have been used a few hours each week, and the spring is still fine after a year and a half... But I'm at the low end of the spring weight range.

The bearings will wear out if they are not occasionally maintained. Because of the bearings, exposure to water sand and grit should be avoided.

The rest of the parts should last for most of the lifetime of the stilts, but may need replacements from heavy usage or incidents that cause specific damage.