Mine was incredibly slow when it first started working, and would fail 50% of the time. Starting last night, it seems to go a lot faster and not fail as much.
I'm pretty sure it will send in the background if you hit 'send' and the hit the home button right after. You won't have to watch and verify that it sends for the whole duration and can move along and do whatever else you need to do on the phone. Texts work this way.
This isn't a direct answer to "how long" it will take, but in case some of you are stuck staring at the screen and waiting.