Converting a statistic to some kind of probability?

Hey all, I'm trying to do some statistical analysis and am hitting a wall with how to approach it.

*This is not for a homework problem, it is my own personal research*

Say I have a historical statistical rate of occurrence of X occurrences per 100,000 hours. (I'm still trying to figure out the rate of occurrence so I don't have the number yet). I'm trying to figure out what the probabilities are that an occurrence happens after say 1 hour, after 1,000 hours, and after 10,000 hours.

Basically I want to be able to have some kind of formula to plug in both an occurrence rate and hours of duration and get some kind of likelihood that something will occur.

Anyway, I'd just like to know how to analyze this. Thanks!