how do you know when

When it looks like dirt, dearie...instead of the organic materials you put in there.

Compost is ready when it stops heating and appears dark and crumbly and no longer resembles the stuff you put in. However you do not need to be in a big hurry to use it though. Most people would use the compost you made this year in the next year. You usually have two compost bins for this purpose, one for this year and one for last year. If not some people just put a pile outside the bin and use it as necessary in the spring.