How long before ovulation does cervical mucus appear?

We're TTC and I had creamy white CM for about a week now.I had an 18 week m/c in April and had two periods since then but they were different length in cycles. I think today I had EWCM but I'm not sure as I didn't have loads. We're bd'ing every night to make sure but I wanted to check when CM changes before ovulation occurs.