Kinda tough to say tbh. Some specs/classes don't begin to shine until they get BWL geared lol. Maybe try a class to 40 first at least since that when you at least get 31 points in your talent tree.

Depends on your time commitment. 20 for a basic knowledge, but I'd say 40 to really know mechanically if you are going to enjoy endgame and the rest of the ride.

Whenever you do Scarlet Monastery I would say. You've done a lot of levelling by then, you've had some World PvP in STV and SM is the first dungeon that can be a bit challenging so you'll get to know the limitations of your class (mana issues for example).

I quit my mage at level 38, so boring.
Spamming frostbolt, drink, repeat.

I know Warlock isn't different but I like the DoT+fear multi-target playstyle.