I saw one study that 20 minutes in direct sunlight creates enough vitamin D3 in the skin to be detected in a blood test 2 weeks later, without any additional UVB exposure. So I think if you can do at least 20-30 as often as you can a week, your chameleon would love it. But more is better when it comes to outside time, as long as they have access to shade.
I will keep mine outside for days or weeks at a time, depending on the weather. The only negative aspect is the temperature, whether it gets too cold or too hot. But as long as you provide shade and lots of misting your chameleon could handle our 80/90F days without problem.
A little wind is not a problem, they have a good grip for a reason. If he is not in a cage just keep an eye out for hawks and other birds that might want to swoop in an grab a free lunch. Just don't leave him unattended and he will be fine. If he is in a cage just make sure it can't be knocked over by wind or anything, so he doesn't make a break for it if it does fall over.
If you are doing a lot of sunshine exposure a week you can cut out the D3 supplement twice monthly if you want, but you don't necessarily have to. Perhaps you can just use it once monthly if you want to. I have almost completely cut it out of my supplement schedule, they get it very rarely.
