8/9 months for server 1 and 2 i think it was

they also take the number of active players on each server into consideration,
so it is possible to have server 5x merged before 4x

generally speaking, the ones at 50 and 70 are better, but you can still draw a few times now to get better ninjas for your team

They do change around the available ninjas in the treasures once in a while