Time Attack mode consists of 20 levels. Each level has a different number of AI opponents and various status effects (such as increased attack or defense, invisible opponents, infinite or zero ultra gauge etc).
Oddly, despite being time-based this is also a survival mode. If you lose a round, that's it - game over. Even if you've got loads of time left.
Also, SFIV is cheap as chips - so yes, you're best off playing as Zangief and spamming left-bumper. That alone won't be enough though, so remember to fire off your ultra and supers whenever possible.
A good way to activate Zangief's ultra/super is to jump at your opponent and do two spins (or more) of your left stick as you are in mid-air. Land and hit LB. This will activate your ultra (if you've got enough guage) or super (if you haven't). A couple of those and those AI pricks are toast.
Look out for a couple of nasty rounds though. Level 14 disables your special moves (no lariat spins for you) but increases your attack. Zangief's heavy punch is enough here and you can still do ultra/super combos.
Level 17 is really hateful. Your opponents have infinite ultras and increased attack and defense. You have to face ten of them. Zangief is 8th, Gouken is 9th (and is the worst kind of bastard) and Sagat is 10th. More lariat nastiness for the win.
Level 20 is the last one and faces you off against 25 enemies. That's the whole roster, ending with Seth. You have increased attack/defense and you start each round with a full super and ultra gauge. If you lose, it's back to the beginning.
Advice for that is be cheap (just like they are), use your ultra midway through the round and then get your super in. Inbetween just lariat for all you are worth.