I've had an instinct solar for 4 months now and I really love it. To be honest, I don't find much utility in the solar charging. I work in an office all day, fwiw, but purposely tested the solar charging capabilities a few times by placing the watch in direct sun light for a few hours and was very underwhelmed.
By my very unscientific figuring, the solar could maybe maintain your battery life if you weren't running any special programs. But the moment you start tracking, using GPS, marking waypoints, etc., you're loosing. That said, I consistently get 15-20 days before needing a charge (varies on what programs I use during the time).
Hope this helps.