How long can you sustain milk supply while pumping?

I have been pumping my breast milk since Oct 31/07 because my daughter was a preemie and she never learned to breast feed properly. Ever since I returned to work at the end of December my milk supply has been slowly decreasing. Luckily I still produce enough to keep her fed, but I'm concerned about how much longer that will keep going. On my days off from work I make sure that I pump every three hours, but on the days I work sometimes I don't pump until about 8 hours later. As long as I keep pumping on my days off and when I get home from work do you think that I will keep producing for a while longer?