I've only done tool and axe handles plus one cedar walking stick, but I usually do a coat a day for a couple of weeks. The rule of thumb is once a day for a week, once a week for a month, once a month for a year then once yearly. That's for tool handles, but should be close for a rifle stock...
I've had mine off several times using a plastic Magpul tool. Sorry you're having such trouble. Maybe destroy the wood and get at the nut with vice grips then replce the nut with a new one. Just spit-balling. Good luck though.