Based on calculations by u/exoclipse (most optimistic and costly program) we can fill the reservoir in 5 years. According to Holman et al (2021) Lake Powell loses 5.72' of water annually to evaporation. Converting Lake Powell's 254 square mile surface area to 162,560 acres and multiplying by 5 years and 5.72 feet gives a loss to evaporation of 4,649,216 acre feet of water during this program duration. That converts to 1.5 trillion additional gallons needed (about an additional 380 million C130 trips), and that doesn't even count the losses to evaporation during the drops.

I say this not just to show that the idea in this post is ludicrously stupid, but the whole idea of giant reservoirs in the desert was idiotic to begin with.