I to the same thing as Italia does. I time freeze them. I take 3 Polar bottles on medium rides and freeze one solid and that will last here in the desert 100+ degree heat for about 2 to 2.5 hours thus I drink that one last; the other bottle I freeze to a slush and that will last about 1.5 to 2 hours, then the final one I just fill with ice and it will last about 1 hour.
But therein lied my problem, I could not be out longer than about 2.5 hours because the water was warm and gone. So I finally caved in to buying a CamelBack 70oz Rogue 2 weeks ago. I tested it this Memorial day and placed 5 ice cubes in it and took the bottles that were full of ice (about a dozen cubes). The temperture that day did not exceed 85 so the water stayed cooler longer. But the weird thing was that the CamelBack with only 5 cubes stayed cooler far longer than the Polar bottles did (the water in the tube got warm though) and with less ice. This weekend will be the big test because temps are suppose to be in the upper 90's and maybe 100's so I will see how cool the CameBack will stay; plus I get to check to see if it will be to hot on my back to wear in these hot temps.