According to a post by Bornakk on the official World of Warcraft forums, the effect of flasks will be reduced to only last one hour instead of two once patch 3.1 goes live. However, all recipes will make two one-hour flasks from the same amount of materials.

Here's the dev team's reasoning behind this update: "Our goal with the change was to allow players greater flexibility when determining how long they plan to raid, as currently we were seeing many players balance time spent raiding around flask duration."

For players who have stockpiled flasks, they can trade two-hour Northrend flasks for two of the one-hour versions. You won't be able to do this with flasks from older content, so you may want to consider that as patch 3.1 approaches. You can discuss the change in this thread in our general forum.