how are refills determined

Hi all. I have been going to pain management doctor for almost a year. I am on mw contain 15mg-3 a day, Norco 10mg-up to 4 a day, and finally goalies 600mg 3 a day. I call in for my refills and they mail the scripts to me as long as I have an appointment scheduled within a 90day window. My question is how do they figure the 30 days for the scripts? sometimes it seems sooner than o thought or later. Example my last script was filled on 5/16/15 for a 30 day supply. When should my next refill date be? they put on the script that it can't be filled before that date. At my last visit in April I explained that my MS contin wasn't helping as much and having tough time sleeping, he gave me flexiril which isn't helping. I would like to increase my MS contain dose as it feels like it isn't helping, any thoughts on the refill process are appreciated. I have neck issues with bulging discs, pinched nerves, bone spurs, and severe degenerative disc disease. Thank you all for your kindness

how are refills determined