Not even that much.
On my old laptop I get a little more than 1000 hash. Assuming that average computer is twice as fast as mine we get 2 khash/s. Inserting it in the calculator [1] shows that it would take 6068212 days on average to calculate one block of 50 bitcoins. That is 332.5 years per bitcoin. At current rate it means $0.02 per year.
