I wash everyday, weather permitting, and it has to be some really nasty cold weather to keep me from jumping in. I have broken the ice on a couple of occasions to get in but conditions have to be right before I will do that. I have found that just stripping down and scrubbing with snow does a pretty good job as well, but I can understand where others would disagree. Another method that works for me is wash the top half and then put on my long silk top and wind shirt, and then wash the bottom half, if it's a bit too chilly for a full dunk.
I spend most of my time above tree line and in some lakes and ponds when I am diving in, I am turning towards shore before I even hit the water. I find that I feel SO much better when I clean up each night both physically and physiologically as well. It is amazing how warm the sun can feel when the water is in the 30's or 40's and the air temp is in the 40's or 50's with no wind. For me the big thing is the wind, and above tree line it is pretty pervasive.
I have had my share of sponge baths, either from the stove or wet wipes, but nothing beats a complete dunk. I don't use sunscreen or insect repellent so I feel a bit better about diving in, but if the body of water or stream flow isn't large enough, then I revert back to the sponge bath. I also will jump in throughout the day as opportunities present themselves. When you are dragging late in the afternoon, a quick dip can get another 3 miles down the trail.
When thru hiking I didn't get as good of a clean up, but found time to wash at least every other day on the PCT. On the CDT the weather was so cold and poor, I felt like I had been bathing all day and didn't want to risk hypothermia for a clean up. I would go for days when I don't think I even worked up a sweat as I couldn't get warm enough even when hiking.