At least 2-3 times a week when not on your period. If you have regular cycles days 10 – 18 are the most fertile time and you can have sex every other night. You can also use an ovulation predictor test and have sex for the three days starting with a positive test.
Don’t douche after intercourse. Nearly all lubricants like KY jelly are harmful to sperm but are not a major factor for most couples, but if you usually use lubricant switch to pre-seed (see link below). I have no commercial interest in pre-seed.
If you have regular periods, you can try for up to a year if you are under 35, six months if 35 or over, before seeing a Fertility specialist (Reproductive Endocrinologist). If you are irregular, or if you are 40 or over, see a Reproductive Endocrinologist soon.