If you wanting regular maintenance for your body and need to work on something specific or have sore points that recur over and over again, once a week or fortnightly is a good amount to start with. Along side massage or reflexology, it is always advisable to have a regular exercise, wind down and stretching routine. After this massage may become less frequent. Shorter sessions are sometimes quite effective but taken regularly to achieve your goal.
If you are looking for general relaxation or a treat for your body once a month is a good amount but thats entirely up to you and your personal enjoyment.