Once you’re eligible (ie. you’ve got the experience, the right qualification/VTAA, clean licence, and DBS), the course to become an MOT Tester is just 4 days.
You’ll usually get your IMI certificate within 2 weeks, and once that’s in hand (and your DBS has cleared), you can book your Demonstration Assessment with DVSA.
So, in real-world terms, you’re probably looking at just a few weeks from starting the course to being ready for final sign-off.