This answer varies for everyone depending on their level of stress, activity, and range of motion. Also depends simply on how much they want to come in. Some clients get a massage once a month while others come in twice a month or more.
When getting a massage it is important for the client to be as comfortable as possible. Therefore, disrobing to their level of comfort is key. Most clients get undressed to their undergarments only, this allows for a more well rounded massage. No matter what the client decides, their entire body will be properly draped other than the part of the body being worked on.
Massage therapy is a service field. So just like you would tip your waitress or dog groomer, you would tip your massage therapist as well. This is greatly appreciated.