For many people, developing guitar calluses takes little more than a week of regular practice. Though this can be a painful process, it usually results in calluses that protect the fingertips and relieve pain.
But if you're like me, getting guitar calluses is not so easy. Even when I'm able to develop decent calluses, almost invariably they disappear after going just a few days without playing. I've found that the best way to develop calluses that last is to practice as long as possible without reaching the point where the skin begins to tear. When you break the skin, there's no going back and overall development will take much longer.
It's also important not to pop any blisters that come from playing guitar. I know it's difficult, but trust me. I've tried everything.
Finally, another thing you can do to promote healthy callus growth is avoid getting your hands wet. Moisture weakens your skin and leads to tearing. The best solution is to use an alcohol-based hand sanitizer instead of washing your hands multiple times daily.
http://www.fingerstrong.com/callus_tips.html
More Guitar Callus Tips can be found here. This website sells a solution that's supposed to help form calluses. But they've also got some great tips and even a video about how to keep your calluses healthy.
