Letrozole tablets help!!!

So my fertility Dr rang me yesterday as it was our phone call appointment and also my birthday, so she told my partner his sperm analysis was normal, all my tests was normal too but still not getting pregnant. So she sent me a prescription today for letrozole to take when my next period is on CD 2 to 6, for upto 4 cycles. Has anyone ever taken these?? How long after taking them did it take you to get pregnant?? Thanx