20 - 30 minutos
Phone used to die after no more than 45 min, even faster if I played Angry Birds, and never discharged below 80%. Then I had to have it plugged in for a few minutes before it powered on, again at 80%.
Pretty smooth, 25 minutes tops. I made sure to note which screw belongs where. Used a vacuum thingie from a bathroom caddy.
Careful when you pull on the LCD/digitizer assembly, takes some effort but don't rush it if you don't want to break something.
P.S. For whatever reason website won't upload my photos, so I'll pop the links: