Rotax say replace the crank in a 2 stroke at 300hrs but its up to you, there is a testing tool that fits in the spark plug hole to check for wear on the crank journals a 5 min job....I have one for my Rotax 503, you can test every 10 hrs or whatever to keep an eye on things.
Most people think the 300hr and replace is very very conservative, Rotax also say to de-coke every 50 hrs but most people owners think 100hrs is more realistic with modern 2 stroke oil, a de-coke should cost around £2-300....if you deem yourself competent you can do it yourself....I do my own, there are some very good step by step DVDs available showing how to take these engines completely apart and re-build with all specs and tools req,
The cost of a completely new 582 is around £3000....not the end of the world....make sure your sitting down when you ask the price of a brand new motor for a 172....a 582 crank at a guess £700.
Nick.