I left it in my DH's car for almost 8 hours and the water stayed cold!! My DH's biggest complaint is that it doesn't hold enough water for him. He wants to know if you're going to come out with a 40oz bottle! Both of my DS's (10 & 11) love it too, after 4 hours of practice they said the water was still icy!! I'm buying 4 more b/c everyone keeps taking mine and I've only had it a week!!!