Check the Runes page on the JoS. The description for Ingwaz states that it stores it until it is needed. You really have only two options, either that the energy doesn't lose its potency over time, or that it loses its potency over time. Regarding the first possible situation, the answer that it stores it indefinitely.
If the second option is correct, there isn't an answer which would tell you that "The Ingwaz rune stores energy for x days", because energy doesn't work that way. If the potency of the energy or its amount weakens, it does so over time.
Why do you ask this, though? Can't you just repeat a working from time to time to ensure it doesn't lose its potency?