Agree with Stigy, there is no way of knowing how long it may be. I passed assessments in October 2018 and DMI in January 2019 for a large depot. I was told in person by the interviewer on the day that it was a really strong interview and that I wouldn't be in the pool long. 18 months later I'm still waiting to hear any more and having just checked the First career portal, the application has now disappeared. I'm not sure if this means I am still in the pool or not.
Luckily, I've been driving with another TOC for over a year now. Best advice I can give is apply for every job you can and don't take being in a pool as confirmation of a job.