I've stored an MHRB solution, and another of A. Confusa for +- six months. I haven't done any chemical analysis on it but the Spice I've extracted after that period of time is still strong and viable, maybe a little less crystalline and more oily.
I din't neutralise them, and I think after that time more alkaoids were extracted, cause I was already at a 2% of yield before leaving it in my drawer for that long.