"Some is better than none."
What does that mean? Well, it varies from person to person. If you are just looking for some occasional relaxation, then a session every 3-6 weeks may be fine for you.
However, if you are looking to address a specific condition, then it is recommended to go more frequently at first and then slowly taper down to a maintenance schedule. Sometimes more frequent 30-minute sessions can be effective until your goals are met and a maintenance schedule is in place. Frequency of sessions should be discussed with your massage therapist after your treatment when he/she has a better hands-on understanding of your particular muscular issues.