OK, 4 hours, 20 minutes, 15 minutes -- how does one decide? I'm planning a trip to the Sierra where the water will most likely be in the 30 degree F range, possibly in the low 40's. How do I determine how long to treat the water?

Maureen: if you are going to boil the water, dont waste a tab on it.

Do you think it's safe to treat water for 30 minutes that you intend to boil for cooking?

I'm a bit more paranoid, especially when hiking in heavily travelled areas or areas where cows or sheep may be present. Given the water temp and the general area, this would be my plan: Fill all water containers at night and disinfect all, especially hydration reservoir (4+ hours overnight) Any empty bottles after breakfast also get filled, disinfected for 4+ hours while hiking, with the exception of any water container on the outside of your pack that will warm with sun and ambient air temp. These get used to replace the hydration reservoir mid-day, if necessary, and are, in a perfect world, refilled and disinfected at the same time. The idea is to always have potable water while also disinfecting the remainder of your water store (if you don't want to carry a lot of water at any given time and water will be readily available on your trip, you can use a UV pen as a primary disinfectant, with chemicals for backup). In more remote areas where the water is clear and your altitude is high (less chance of upstream contamination), 30 minutes would be sufficient, for me. I have never gotten sick through this practice, which is the whole idea of disinfection. Others, like Randy, have probably not gotten sick with their practice either, so it's a matter of choice, risk and your own personal paranoia scale. I would never go less than 30 minutes and no one I know would either.

30 minutes if it is clear, 4 hours if it is dirty.