This is the method I use. I prefer stock TTs like current ones. If it is a tuned TT, I normally wait for it to settle down a bit so fastest car gets found for combo and there are usually good tunes (Normally Active-Steering ones) which can then be tweaked (I drive without Active Steering) quite quickly to be competitive.
Before I run TT, I usually watch leaders ghost and look at speeds in corners and ideal racing line. Then I do a few laps myself, normally quite easy to get in top 100 or top 50 after a few laps. Then I watch replay of fastest leaderboard lap, confirm some things and then go back time trialling. Depending on how competitive leaderboard is, top 10-20 is usually within reach within an hour on a matured leaderboard. However you can spend a few more hours trying to nail a lap and hit close to best sectors which normally doesn't really happen unless I get lucky. Generally improve by a few tenths after say an extra hour but potential is usually a lot higher.
Tuned leaderboards, you can spend more time trying to be competitive as you need to get car setup and feel it is competitive. I remember when I borrowed CargoRats 2J (Thanks again) for Tsukuba and was challenging and getting number 1 position for a bit but did not run any laps after. It was good enough for 3rd in the end and fastest without active steering and I felt there was the potential to do the number 1 time if I spent a good few hours on it and tried to get closer to my best sector times.
About ghosts, I personally don't use other peoples ghosts, just use my best lap or close to my best lap and use that as reference. If I am chasing ghost, I am more likely to be behind it at end of lap. If I am leaving my ghost for dead, I am more likely to improve

.
I am far from fastest people in GT though. I would imagine the top players like Ti-tech85 would probably be challenging or getting number 1 times within 10 minutes or at least within half an hour. Towards the last week or last few days of TT, it would be probably hard to get number 1 time that quick though even for an alien of his level so a few hours will probably be enough. That will still be impressive as people setting fast times are quite alien too and also probably put quite a bit more time into it.
Finally about learning fastest way yourselves, you get used to that already and have a good idea of it before checking someones lap out as you get closer to the top. I think this comment by Lewis Hamilton fits well:
Lewis Hamilton
"You know how in 'The Matrix,' he can see the matrix? When I'm driving, I see the lines."
Tell that to DHolland...lol..
I think Dan thinks Arnaud is fastest GT player.