No seats it's all grass. There are five stages spread across a large field. About two days before the show they will post the set times at www.coachella.com and they do stick to them pretty rigidly so you'll know when to shoot for. Arriving that late you shouldn't have long traffic lines but you will have a long walk as you'll be way way out at the end of the parking. I'd aim to get there well before set time, especially if you want to muscle up to the front.
(Secret tip, go round the left-hand side of the main stage, everyone drifts in from the right and you can get much closer on the left)
However, if you can work some earplugs or something, I'd urge you to go for the whole day (or weekend), it really is an awesome festival.
Either way, enjoy it.
PS the "seat number" on your ticket is for admin purposes only, it fools a lot of people.