How often do you get a massage? Just like chiropractic care you might need more massages closer together in the beginning of care, and when you are in the maintenance care once per month is the norm.
Is it necessary? I would put massage and chiropractic adjustments as my top 2 self care visits per month. Your body is the only one you get in this lifetime. I would do what you need to do to keep it moving optimally.
When should I get one (before or after an adjustment)? I don't feel it matters which way I get these bodyworks done, but I always make sure I get adjusted when I get a massage. If your body is protecting a subluxation and you don't take care of the subluxation then your body will get too loose in an area that needs protecting and you can feel much more sore after a massage.
What type of massage do you need? I would highly recommend a therapeutic massage vs a relaxation massage when you are trying to treat issues in your body.