How long can Fioricet be taken?

I've been taking it on and off for 12 days, now. Some days, I've taken up to 6 tablets. I have constant pressure behind my eyes, with reoccurring headaches. I'm getting a spinal tap done tomorrow morning and an MRI Friday. Having the constant pressure and stabbing pains in my eyes makes me want to just keep taking them, but I know they're habit forming. That, and I'm weary about taking anything with acetaminophen for long periods of time. The neurologist said that it was ok to keep taking, as long as it wasn't more than 6 tablets a day (and taken at least 4 hours apart). He also stressed that I could become dependant, and to only take when I have to. I've actually avoided taking this when having bad headaches the last couple of days. I'm a paranoid person, but anything that deals with prescribed meds, gives me a justified reason to be.