It is impossible to over state how "useful" it is to work the ranges. In any match up, you've got some sweet spots, some spots where you break even, and some where you give up your edge. When you win innitive, you want to hit your sweet spots, or at least set up to hit it next time, and when you lose it, you want to try and prevent your enemy from the same and force a break even. Sometimes, for a short ranged mech the sweet spot is 0-3, some times for a sniper its 24-27, sometimes its about a very narrow band, sometimes its about getting all your weapons to work at once (6 hexes for LRMs and MLs, and the AC20 as on the Atlas, or 3 hexes for MLs and GRs or old PPCs). But there's always a sweet spot, we you've got the edge over your opponent, and if you can get in it and stay in it, you'll surely win.
As for time, if you know your ranges and your opponents' ranges pretty well, and you can count reasonably quickly, it is usualy pretty quick to move even if you have to scope things out for range brackets, at least for me.