just as a passenger with no gadgets to measure these sorts of things--
I often look at my watch and time how long it takes to wheels up in various aircraft, counted from engine spinup. I would guess that wheels up happens at 150mph for small jets, heavier ones it's more like 200mph. Times are pretty unsurprising: 20-25 seconds for an RJ, 30 seconds or so for 737, a little more for a 757. Fully loaded 747 going to SYD, almost a full minute.