The challenge log is VERY useful. It resets weekly (and you don't need to do anything special once you've picked it up, you'll finish stuff automatically after Tuesday reset) and you get tons of exp for completing the challenges (5 FATEs in different regions, 20 levequests, stuff like that...nothing grindy) - checking on my level 30 BRD, I would get 24,375 exp for each of the two FATE challenges (Highest rating in 5 FATEs in 5 different regions & complete 10 unique FATEs). The exp adjusts to your level - for the same challenges, I'd get 48k each on my level 40 PLD and 5.3k on my level 15 lancer. You unlock it through the level 15 quest "Rising to the Challenge" at Limsa Lominsa Upper Decks (x11,y10) - the NPC is I'tolwann.
There is no way to see in-game how long you've played.
The achievement vendor is in Apkallu Falls in Old Gridania. His name is Jonathas, Master of the Rolls. However, you get titles immediately upon getting an achievement - they're in your character menu under the profile tab. You click the little triangle to the left of "Title" at the top of that tab.
On each class, or just on one of them? If you've only done it on one class, you should do it on the others, too - especially the lancer. It really helps at those lower levels!
