I can keep clones/clone mothers for as long as I want. I bonsai the mothers and keep them small with constant trimming and lower levels of nutes and light until I want to do a clone run of that strain, then feed up the mom, increase light, let her grow bigger, then take the cuttings I want then I ramp her back down til I want more clones from her.
Check out my cloning thread and look for the keeping of clone mothers.
I’m on year 3 of keeping a Cherry Ice Cream clone and I had other clone mothers for even longer than that in the past.