How often should I practice guitar each week and for how long do I play each session?
In order to develop new skills and get good at anything, you must do it a lot. If you want to progress as a guitar player, then you need to get your instrument in your hands every day. You don't need to practice for hours on end, but you ought to get in at least 30 minutes each day. It doesn't have to be all at once. Your practice time can be broken up into smaller sessions with 10-15 minutes here, 10-15 minutes there.
It's inevitable that you'll miss a day now and then. But if you plan on playing guitar every day, then you're sure to cover most of them.
If you should ever have extra time, then certainly you can practice for longer periods. Just be sure to get some playing in even on busy days. Grabbing your guitar before you go to bed and reviewing things for 5 minutes will at least help you to retain what you've learned and keep your finger calluses from getting soft.
Another key to successful guitar playing and productive practicing is getting involved with some type of guitar activity or commitment. For example, schedule jam sessions with other musicians, start a band, book a gig, study with an instructor, take a guitar theory class, play at church, or any other opportunity you can participate in. These ideas will not only increase the time you spend with your guitar in hand, but you'll also enjoy the social interaction and fellowship.