It depends. I'd say if you're going to get the cheap $5 ones off e-bay, it only lasts AT MOST until early Spring iif you put it on top of your grips with the shrink wrap. I heard if you put it under your grips, you barely feel them and its also a ***** to do. If you put the heated grips on top, you're going to get sticky, slimey, glue on your grips when you remove it.
No, the cheap eBay grip heaters are intended to be installed under the existing grip; in other words, between the bar and the existing grip. The shrink wrap is used on the clutch side as insulation between the bar and the heater. The heater element on the throttle side should be wrapped around the plastic throttle tube, no insulation required. Typically with this type of heated grip, the throttle side will be warmer than the clutch side, as the steel bar acts as a heat sink, despite the shrink wrap insulation. For five bucks and free shipping, though? Give 'em a try!
I use Heat Demons by Symtec. About $50 from GP Bikes. My clutch side side isn't quite as warm as the throttle, but it's acceptable to me. The Oxfords are likely the best solution, even at about $100. I chose the under-the-grip style because I wanted to retain my stock grips, and above all, have a discreet installation. I was able to hide the switch, so most folks don't know they're there. The Oxfords use their own grips and have a larger switch.
Heat Demon:
Oxford: