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 the best monthly casino bonuses, you really need to look at the VIP schemes. They offer bespoke bonuses to regular players every month.
Here is a list of regular monthly bonuses/ reload bonuses. Please check if they are available for roulette as the rules often change.