I have found upgrades on SFO-JFK to be hit or miss. Sometimes I can get an upgrade immediately at booking, other times it clears days before, sometimes the day of, and sometimes it never clears. (UA Gold Member) Early morning flights seem to be easier to get than afternoon flights, and red-eyes seem near impossible.
I recently requested an upgrade on a JFK-LAX flight (only 17,500 miles!) a month out, and my upgrade cleared after about a week (about 3 weeks before my flight). Never got an email, just noticed in my reservations when I checking to see how many seats empty. Mid-afternoon flight on Friday, so I was a bit surprised!
I find it completely worth the 17,500-20,000 miles for these 6ish hour flights. I usually work on flights, so I generally don't sleep. But being able to raise the footrest independently is amazing. It makes me feel like I am sitting in bed on my laptop. There is also plenty of space for me to have out a few sets of papers, too.
I wish all first class seats had this footrest option. While I generally try to fly first when it makes sense based on cost/miles, E+ already gives me the space I need-I am small-framed and so short that my feet don't even reach the floor in E+ unless I slouch a lot. I end up taking my shoes off and putting my carry on under my feet as a footrest in E+ or in first (for those who are not vertically challenged, it is not comfortable when your legs dangle). I'd be more motivated to pay for an upgrade to first if I could put my feet up on a footrest in non-p.s. flights.