Terry,
I do the same in the winter, when the water temp drops below 60, I go to 2 hours a day of pump run time with a SWG setting of 45%. This gives me about 0.5 ppm. You may not need to run the SWG at a higher % since the pool will probably require less chlorine with lower temps. Last year during our winter, I kept the chlorine between 0.5 and 1 ppm and did not have any problems but your conditions could be different.
As for SWG life, as Waste pointed out, it depends on run time per day. So,
3 hrs @ 90% = 2.7 hrs SWG run time
6 hrs @ 45% = 2.7 hrs SWG run time
9 hrs @ 30% = 2.7 hrs SWG run time
Each of these settings combinations will produce the same chlorine and have the same effect on cell life. But as I mentioned above, when water temp drops, you will need much less chlorine in the pool so even though you are reducing pump run time, you may not need to increase SWG % as much as you think or at all.
Bookmarks