I co-wash everyday, and shampoo whenever I feel like it, which might be as much as twice a week sometimes. (Though, I always mix my shampoo with a bit of conditioner.) I am pretty sure we have hard water here, but I wouldn't know the difference. I haven't seen any bad results from it. I look forward to cowashing everyday. I don't get tangles and I don't lose clumps of hair like I used to when I washed much less frequently. I almost never ever have difficult knots in my hair now.
I am asking my husband to look into buying a shower filter so I can see if there is any difference. But I plan to keep this up throughout the summer. It feels wonderful to have clean hair each day. My hair does go days without being dry, unless I airdry before putting it in a bun, but that's fine with me. I don't know what I'll do when the whether gets cold. I long time ago I used to go out with a wet head every morning in the winter and it never created a problem, so I might just continue on like this.
I am transitioning, btw.