How often are new expansions?

I joined during the WoW Mercenaries promotion and I’m wondering how long before that the latest expansion was released as I’m wondering if it’ll be easier or the same difficulty next expansion to progress the rewards track.

It gets easier.
You will have about 4 months until next expansion when Fractured in Alterac Valley has been released.

thanks. I think I have 13 levels to go (to 100) which at one a day isn’t terrible but also far from optimal.