In order to answer this, we really need to take in to account what has brought someone to get a massage in the first place.
For those using massage for general wellness, I recommend a frequency of once/twice a month, which allows people in normal good health to receive maximum benefits as each session builds upon the positives of the last. Bodies used to massage respond to it better and faster to it than those who have not experienced it before or in some time.
Research demonstrates very strongly that massage helps to combat depression, stress, anxiety and insomnia-related-to-stress, so for people going through a particularly difficult life situation or for those in high-stress professions, I suggest aiming to have a massage every one to two weeks.
For the treatment of a chronic condition eg arthritis, or for an acute problem eg muscular injury or postural aches, people generally need to receive massage more frequently in order to see any real improvement. I advise my clients individually on the type of schedule I believe most suits their needs, but very broadly speaking: approx once/twice weekly for the first four weeks, then gradually increasing time between sessions as symptoms improve, before moving down to a maintenance schedule. These 'decreasing intensity' schedules are most likely to have the best results on improving mobility or painful symptoms. You wouldn't expect to reach peak fitness after one exercise session, the same is true in harnessing the benefits of massage.
With pregnancy massage, it very much depends on the individual progress of the pregnancy and the mother's health in response to it, but many women find that in the last trimester particularly, weekly massage helps to improve stress levels relating to the impending birth and can certainly alleviate common pregnancy symptoms such as swollen ankles, back ache, headaches and sciatica.
For those aiming to enhance athletic performance, massage is a superb support mechanism for reaching peak physical fitness whilst providing some level of injury protection. People heavily involved in sports recreationally, or those competing at a serious level professionally are advised to have a massage twice (or more) a week during periods of intense training and perhaps once or twice a month during their off-season to maintain muscle health and flexibility.
The bottom line is that people who wish to treat specific conditions or symptoms will need to have massage more frequently than people who use it purely for enjoyment and relaxation. Not everyone has the time or financial ability to commit to 'ideal massage schedules' however, so to them I say "Some is always better than none!" Ask yourself how you felt after your last massage... When you need that feeling again, book yourself in.
NB: Manual Lymphatic Drainage Massage is a different kettle of fish altogether and the above guidelines are not directly applicable to it.