Most casinos give monthly or reload bonuses. These are not always publicised and are often dependent on how much you play. There are some that will just give every player an amount though regardless of the amount of play.
To get the best monthly casino bonuses, you really need to look at the VIP schemes. They offer bespoke bonuses to regular players every month.
Monthly bonuses have fallen out of favour recently as they attract matched bettors who try game the system.