events begin approximately every 3 weeks. they last about 2 weeks, with a week in between. completing area 50 in the event adventure will unlock the champion. the first event for console will be Wintershield Year 2 and will include two unlockable champions.
you can also unlock event champions through Time Gates, which open for 72 hours, every three weeks (during the off-event week). you are offered 3 champions to unlock or earn gear for and there is a guarantee to have at least one you have not yet unlocked.
this works out to about 3 champs every month. locked champions are available for purchase with cash in the store if as well.