Officially, Philips states that the blades last around 4 months (* For an optimal shaving result. Based on two full shaves per week. Actual results may vary).
I bought my device in October 2016 and am still on the first blade. I have a classic 3-day beard which I trim to 3mm 3-4 times a week and I haven't noticed any deterioration of the blade.

Probably forever, the first one is still in there.