It depends on how well they were packaged to begin with. If they were vacuum-packed or nitrogen-flushed, they should have minimal degradation. If they had lots of exposure to oxygen prior to or during packaging, they'll degrade quicker. The calculator Steve posted a link to is one of the better ones because it takes those variables into consideration.
I notice you said 'refrigerated', not 'frozen'. The colder you keep your hops, the better. Store them in the freezer if possible.
FWIW, I recently brewed with vacuum-packed and frozen Nugget pellets that I'd had for at least 18 months. They still had good aroma to them, and the bittering was fine. It wasn't a particularly hoppy beer, so YMMV.