bk-MDMA depletes serotonin (though, not to the same extent as MDMA) so the rather than being concerned about a tolerance build, you should concern yourself with spacing for the sake of your health and receptors. "Tolerance" is something I'd rather associate with something like weed or alcohol, not something than can (realistically) deplete a vital neurochemical.
I'm not extremely experienced with methylone, but my friends who are regular users space it out every 2 weeks if not longer as a rule. Most people report that tolerance building is not a big issue with methylone, but you certainly should not test that out.