How far in advance can I request a leave of absence from my civilian job as a reservist who is deploying?

ESGR is going to be your best resource for this kind of question. http://www.esgr.mil They have a link for each state with local contacts for you.

I'm going through this now as well. I already told my employer as a heads-up and my unit doesn't mobilize for another 6 months. It is a hassle for me to take off though (I'm a teacher/coach and have to go through a whole leave request process to find a replacement) but they're working with me so far even though I don't have individual orders yet. My job doesn't start the leave until the day the orders start, so in the past I've had to take personal days in order to travel to the duty station. I think it depends on what kind of job you have and frankly how nice of a boss you have.

You will want to have orders in hand before making this known to your employer, or you risk losing USSERA protection.
The law requires you to inform your employer within a "reasonable" time frame so that they can take any necessary measures to prepare for you absence. That is undefined in the law, but for a deployment, 90 days is a reasonable amount of time in my view.