This answer will probably vary wildly. Ten blades will last me about a year. I average about 4 shaves a week, and I get over 20 shaves per blade.
YMMV as razor, number of passes, whether you shave everyday, and most importantly technique play a big factor. When I first started, I was over thinking the blade. Now I am much more comfortable with my technique and it leads to blades lasting much longer.
I used to install a new blade every week, but I've found that I can get down to two blades a month without sacrifice using Astra SP. I suspect that I could do this with other blades as well, but I would have to go back and revisit. The Brazilian Gillette Platinum Plus blades that were available at Walgreens last a month for me, but I think that's exceptional.
So with the Astra SP, five months. But I don't try to get super close with multiple passes.