I use a Thermolution in the PNW's 45-55F water year round. Most of the time I turn it on about half way into a dive as I start to get a bit chilled. If I'm doing a Third dive of the day I tend to turn it on as soon as I splash. 90% of the time it stays on the low setting, but for those extra cold 45F dives it gets bumped up on the last dive for sure. It think it does a very good job of taking the edge of any discomfort. The batteries are easily good for 3+ hours on a low setting. I've never exhausted the batteries so I don't really know how long it will go.