How far ahead can I submit my terminal leave package ?

You're supposed to look for jobs long before you get out. If you gon on leave and look for jobs, you're about a year behind the power curve.
You should be in SFL TAP right now and they'll explain all that to you. You can take as much leave as you have, it gets turned in with your ETS packet.
What's SLA?

You're a year out from ETS. Start looking for jobs now. If you're not on LinkedIn, get on LinkedIn. If you are, connect with me, if you'd like. RallyPoint has jobs advertised as well. Consider selling sixty days of leave. That will put an extra $4k or so in your pocket, which helps the transition. Use the rest of the leave as needed for job interviews and such.
Start talking to your leadership now about your transition plans. Ask them when you should submit the terminal leave packet. Six months out is probably good. If you sell 60 days you'll have 38 days left and that will require a BC signature, I believe. Which means it has to go through about six levels of leadership, and you know how that can go.