In short, you should get massaged whenever you feel like you need it.
Wow, maybe I should go into politics with a response like that!
Honestly, it depends on many different factors, including the severity of the issues in your tissues, how well you're taking care of yourself post-massage, and your daily routine. For example, if you get a massage initially to combat the "tech neck" or "office slump," it depends on how much you are stretching out your pecs, if you're consciously rolling your shoulders back in order to correct your posture, and as always, if you're drinking enough water!!
If you recently started working out, we would recommend taking care of your muscles post-workout with foam rolling and some good old-fashioned stretching, in addition to weekly massages until the DOMS starts to lessen or becomes less debilitating.
If you are looking for a way to combat stress (a more relaxing massage), start off with monthly massages and see how that works for you, but if you aren't changing your daily habits that are contributing to your stress, you may not see results unless you get bi-weekly (or even weekly) massages.
We do have clients that stay active enough (with proper aftercare) to require weekly massages and we'd still like to see them more often just to feel a change in the muscle stiffness.
Aftercare is the most important deciding factor as to how often you receive massage, regardless of the type of massage you're looking for.
As always, communicate with your therapist openly and honestly about your goals for treatment and they will be happy to discuss options with you.