The good thing about the Ring is that it's split into very distinct sections, so for such a long, busy track it's very easy to learn. You've got to break it down into individual tracks, concentrating on them one-by-one.
There's an abridged version of the Isle of Man Mountain course on Grand Prix Legends, and a tool for it which allows you to start at different points, so I started before the final sector and when driving I'd keep edging into the first sector of the next lap, learning it bit by bit too. Then I'd move back a sector and do that a few times, carrying on into the final sector, then again the start of the next lap.
This was for a league race and I started months in advance, spending about 30 minutes a week learning it sector by sector. I didn't do a complete lap until the day before the race.
I uploaded a video last year, as it's certainly one of the best laps I've done in GPL. You can see how well I knew the track and that was only spending a total of about 2 hours split over several weeks. I knew it intimately, which kerbs I could ride and so on.
It's easier to learn the Ring due to the way it's set out and the flow from one bit to the next. It's much harder to just turn up and blap around lap after lap - instead try to break it down in your head.