Say you get to the limit exchange Regalias (20 or 30 per month I think), after so you'll need to summon for Regalias since you'll have the limit on the shop, a natural month has 27 days and you need 120 Regalias for a MLB (4) Weapons, this says that you can either get 101 or 111 Regalias per month if you farm daily, thus it should take 1 month and 15 days or so, 2 months if you slack like me