Most of the winter except for possibly one month, such as January. Basically until it gets so cold and rainy that it is unbearable for my wife to go out and do her therapy exercises and swimming. Nevertheless, as bad as that is, she would rather do that then use the indoor community center pool with it's 10-20 times higher active chlorine level, restricted hours, and long walks to access.
For most people, the weather in our area turns too cold around mid-October to be practical to swim again until around May, but there are days when the weather is not rainy and not so cold as to be unbearable. I put the following chart in
another post that I'm copying here:
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
.56..62..64...68..74..78...81..81..80...75..64..56 ... Avg. High Temp
.40..44..44...45..48..52...54..54..54...50..46..41 ... Avg. Low Temp
.49..54..55...58..62..66...69..69..68...64..56..50 ... Average Temp (slightly more weight to daytime temp)
7.9 5.8 4.9 2.0 . 0.3 0.2 0.1 0.1 . 0.4 2.1 5.6 6.3 ... Average monthly rainfall in inches
3.1 3.9 5.0 6.2 . 6.8 7.0 7.3 6.9 . 6.2 5.0 3.5 2.9 ... kWh/m2/day from the sun (30-year average for month; panels facing south tilted at 38-15=23 degrees)
. - .. - .. - ... 6 ... 2 .. 1 .. 0 .. 0 ... 3 .. 8 . 11 .. - ... Gas Avg. Therms/day used for pool heating (1.67 Therms input per ºF increase)
. - .. - .. - . 300 100 50 . 0 . 0 . 150 400 550 . - ... $ Cost @ $1.68/Therm
So extend this into the other months at around $700-800 per month in early December, February and March with it not heated only in late December (i.e. during the holidays when we are often away anyway) through January, but remember that the pool is heated to 87-88ºF as it is used for exercise therapy with only some swimming. It's really late December through January that the temperatures are the coldest. October through April is more about avoiding the rainy days.