Around 25-40 minutes I'd say (though lately ranked games seem to have been averaging 40 minutes for me)
The meta: Darius. Well.. Not completely, but generally if you get Darius and you don't win, you either got countered hard and the rest of your team can't pick up the slack or you just are no good with him lmao. But yeah, for the most part it's bruisers. A fair bit of APC champs can do okay, just have to be built with more bulk in mind, and adcs are wrecked for the most part, you pretty much need a great escape (corki, ez) Supports don't really happen at all, I once had a soraka on my team, and it wasn't the worst thing ever, but there are definitely better picks.
Also lane compisition is generally 1 top 2 bottom, or 1 top, 1 bot, 1 jungle(or roamer as some refer to it as) all depends on your play style and how you want to try and control the map.
If you like a big bruiser brawl then you will have tons of fun, and if you like seeing darius and singed banned in every game you'll have even more fun. The game has a little less focus on farming and laning, though it's still there, TT, to me anyways, is a little more objective oriented (altar control, overall map control, and vilemaw being right in the middle as to not be biased to one side). And well, if you hate ragers in solo, and play ranked I have good news for you, there is NO solo queue. That's also bad news too because it means unless you want to play with baddies/ragers/trolls all day in normal games you'll have to find yourself a team. Fortunately I was able to find mine easily since two of my good friends love the game mode almost as much as me. Also it hurts that there isn't even a normal draft pick option for 3s.
Also if you want to know more about the game mode in general and the meta check out the guides on dominatett.com they currently have two tier lists and an overall beginners guide I highly recommend it for anyone starting out on tt.