I have to be honest, I use my treadmill so little in the summer time that a layer of dust forms over it! The most use that it gets is when I need to dry out something like a tent. (In case you hadn't already discovered that exercise equipment makes a great clothes rack!)
I don't feel particularlly guilty about this though. I get plenty of activity outside playing with the kids and looking after the property. My lawn also needs to be mowed at least twice a week, which gives me another 20 minute workout.
So time for the treadmill is few and far between. I do try and get outside for a run at least 2 times a week. You would think that with all the extra daylight in the summer I could find more time for that? I guess running around after the kids counts too!
It is more important however to keep track of your exercise and make sure that you are getting at least 3 periods of activity a week that will keep your heartrate in your target range. Perhaps it is swimming, playing soccer with the kids, a hike, walk, run, golf etc etc. The summer offers so many different options that don't involve being stationary on a treadmill.
For me personally I look back at the time that I spent on the treadmill over the long winter and reap the rewards! I love to golf and having the same energy level at the end of 18 holes as at the first is thanks to the time on the treadmill.
Hey – don't forget though that the summer time is also a great time to get your exercise equipment serviced!