It is literally a quick multiple choice thing regarding whether you've sat tests before, whether you have safety crit experience and your location. Shouldn't take anyone more than 10 minutes unless you're working out exact timings for the relevant questions, even then, call it 20!
Regarding the other queries on here, only tests done at an OPC centre (or TOC centre) count towards lives, anything sat at home does not.
You can apply for other roles within 6monhs of sitting OPC tests, but cannot sit any other tests within the 6 month period. This is so you are not able to remember the tests exactly as they were (though I would add they were very familiar even 2 years late for me). So if you sat a CBI within the last 6 months and Chiltern use MMI or vice versa you may not proceed at the point. Depending on how long the Chiltern process is of course