A lifetime, or long enough that it would feel like a lifetime. The real question to ask is that how you really want to spend your life? The fact that you're asking makes me think you've already chosen another path.
It does not take a lifetime. It took me about 6 months to cover upper-division electricity and magnetism and quantum mechanics, which is probably all that you would need to get the most basic but complete understanding of this paper.
Climbing to 20,000 ft from the 16,000 ft makes you think that climbing mountains is easier than it's going to be for the person at sea-level that you're advising. If you've been at 16,000 ft for some time even just that acclimatisation is going to make a huge difference.