10 - 30 minutos
Battery on my iPhone 4S was getting very weak after 2 or 3 years of use.
Very easy to do. No problems at all. Took about 20 minutes.
I think that although a new battery may have provided some small improvement in battery life, I am still seeing the battery last fewer than 12 hours or so. It may be that by running advanced versions of iOs on an older handset I am destined to see really bad battery life, especially with heavy use.