fair enough, sry I was rough. An estimate...

From above:
~10% of the time you flop a straight draw
~10% of the time you flop a flush draw

so 90% of the time you don't get a flush draw, 90% of the time you don't get a straight draw. .9*.9 = .81, so 81% of the time you get neither.

That means 19% of the time you flop a straight draw or a flush draw, or both. This also implies 1% of the time you flop a straight flush draw.