Ok hard data here. Filmed it using my iPad time-lapse mode.
Completely dead iPhone SE plugged into my iPad Pro 12W charger brick. Plugged in at 10:01am, actually powered on (auto power on from plugging it in) at 10:02am. I set the screen brightness to the lowest level but left the screen on to film the charge progress (so it might have charged slightly slower then if the screen was off)
Anyway, hit 100% charge at 12:15.
So thats 134 minutes from empty to full using a 12w iPad Pro charger. (with screen on but at lowest brightness)
I'll run it down again and try the same experiment with the included 1amp charger in a few days when the battery dies again. I love the longevity of the SE battery compared to my old 6!
UPDATE: Ran the test again today, but using the included 5W 1amp charger. Took almost the same amount of time, only about 20-25 minutes longer. This time there was a big difference in time from when I plugged it in, and when it powered on finally (it must have been really dead.) Last time it went from flashing the "plug me in" graphic to powering on in about 2 minutes. This time it took almost 7 minutes to power on. Once on it was set with the screen at minimum brightness, but set on all the time so the video camera could capture the charge progress.
I also forgot I had a watts-up thing, and this time the charger was plugged into that also. The 1 amp charger pulled on average 4-4.2W for the bulk of the charge, then started to taper off at 70%, and was about 1.8-2W from 90-100%.
So it looks (to me at least) after testing that the iPhone SE doesn't support charging above 1amps. The 12W iPad Pro charger did charge it slightly faster, but we're talking 2 hours 14 minutes with that, vs 2 hours 40 minutes with the included smaller 1amp charger. Perhaps the 12W 2.4amp iPad Pro charger is a more efficient charger and can output the true max of 5W for the iPhone SE? (because it has some overhead room not being used? I dunno) I'm probably going to run this test one more time using the iPad Pro charger but this time plugged into the Watts-up thing and test that theory. I think it will feed the iPhone SE a solid 5W for the bulk of the charge (before the taper at 70%) vs the 4.2W the standard charger puts out. That small difference probably makes the iPad Pro charger charge up the iPhone SE a few minutes faster.
Interesting footnote though. For ***** and giggles (and because it was dead) I did the same test using my old iPhone 6 on the 12W iPad Pro charger, but this time also plugged into the watts-up as well. I should mention my iPhone 6 battery is terrible and is in no way representative of a normal battery. It's crap battery performance is part of why I bought a SE. According to iBackupBot it's cycle count is 478. <See attached screen shot>
Anyway the iPad Pro charger pulled 5.5-6.1W for the bulk of the charge, and also started to taper at about 70%, pulling 2.5-3W at the end to 100%. Dead to 100% charge in 1 hour 49 minutes.
/edit corrected my times after reviewing the time-lapse video of the charges.