Indeed, it's difficult to see how our heroes could have maintained phaser contact with the Scimitar
, which was cloaked for the most part, maneuvered quite a lot, and was firing back so hard that the heroes had to maneuver, too. Most of the shots fired by the E-E were "ranging shots" anyway, attempts at establishing where the invisible enemy was, and any sustaining there would have meant wasted energy.
Apart from that, though, the standard mode of phasers in TNG/DS9/VOY era ship-to-ship combat does seem to be to dwell the beam at the target for as long as possible. It's just that in TNG, our heroes usually are so superior in firepower that they don't need a long duration beam to defeat their puny opponents - especially when "defeat" is defined as silencing the enemy guns while minimizing death and destruction. In DS9, a beam is often sustained until it starts coming out the other side of the target ship...