In the 737, provided you are ahead of the game with a good plan and no other restrictions you can sometimes do a thrust levers closed descent all the way from cruise. It requires 'managing' the energy of the airplane. If ATC leaves you high for a moment, get the speed back to 210 and as soon as you are cleared further down 'dive' it down by winding up the speed. If all works out well, as long as you are wel below the glide slope, you can comfortably start taking flaps 1 and reduce to 190 kts or so at around 10 miles from the threshold. As long as you are at flaps 5 and 180 by the time you intercept the glide things will work out well, unless you have a strong tail wind and a full passenger load. Gear down around 1600 RADALT or so (+/- 5 miles) and get the rest of the flaps out. Our company policy used to be stable at minimum 800 RADALT. Using 210 knots at 20 miles as a gate is too conservative and may adversely affect traffic flows and upset ATC. You just drag your feet then, waste fuel and upset the other guy on the flightdeck. A good gate to use is being 210 knots at no less than 12nm from touchdown and then further reducing it. Gear down at around 2000 AGL should work well, but take into consideration headwind/tailwind and traffic ahead of you to land or depart (use TCAS). There is nothing more satisfying then doing a thrust levers closed descent and approach with the power coming on just when you got all the flaps out, nice and stable around 1000 RADALT. Very efficient, fuel wise and time wise and comfortable for the pax.