Having the ability to hover means you are able to fly as slow as you want, all the way down to zero. As far as practical/operational instrument airspeed It’s aircraft and SOP dependent. We typically flew the approach at 90 KIAS in the H-1. Cruising IFR we’d pull around 120 KIAS. You can fly around at 5 knots if you were so inclined, but there isn’t much utility in it.