Ah the beloved immigration and checkpoint authorities, what could I possibly say in their praise. Anyways, if you're one of those lucky blokes who doesn't need a US visa, rejoice you're not missing out on any worthwhile experience. Though you too will need to take off your shoes at the airport ... that's just the way it is.
So now if you do need a visa, what you really need to prove to them is that you've got all the intention of coming back from US. So you need to show them you've got enough money to make the trip. You'll also need a letter from your school/university stating that you're a student there and a letter from Apple, inviting you to attend the conference.
The time for the whole process depends on where you're from and where you're applying from. Since this is different for each nationality and the process for each jurisdiction of application is different I can't go into great details for this here.
This I believe is the most tedious part of getting yourself to the other side. You can't really be certain of the outcome here either way. so, GOOD LUCK!