Today I have spent the whole day mining at the same address with two i7 PCs, one 8-core and one 4-core, and have managed not to find a single block. Not even one. This is really throwing away electricity, these powerful CPUs consume a lot and in my country electricity is expensive like hell. I will give it a few hours more but at some point I'll have to cut the losses and stop mining. It will be cheaper to buy the coins mined by others.

EDIT: After 14 hours I have finally found a block. So I will keep the PC on