Hi. Hope the letrozole was prescribed by an gynaec after your initial harmonal checkups. Ovulation in an induced cycle should occur around the 14th day of cycle, however some times delayed ovulation upto 16-18 days may be possible. I would recommend a proper follicular study ( done by ultrasound) to know the exact stage of your follicles.
Next Steps
Visit a gyneac for follicular study ultrasound.