Ovulation testing once or twice a day?

[allthingssparkly]

I started ovulation testing yesterday and BBT as of this morning. I'm currently CD9, Pre Mom is predicting my fertile window will start in two days. When should I start opk testing twice a day instead of just the once which I'm doing currently?!

I was testing twice a day in my ovulation period predicting by Flo - never got a positive test (could see line getting darker but never positive) but pregnant! I worried so much we weren't ovulating because of the tests, but worked out for us in the end