I'd agree with what our esteemed moderator has said.
Low, about 4 knots. High, about 6 knots. Subject of course to weather being higher or lower than usual. With GPS, you'll know your speed over the bottom. With a wood or stryofoam chip, timed from bow to stern, you'll know what your speed through the water is. They you'll know if you're sailing in current.
But our estimates on the web don't mean much to you on the water, you'll have to take both an over-ground and a through-water speed, then compare to see if you are in a current and if so what its direction is.