With the bug fix to the rate in which a Pokemon’s motivation decays it has now been observed that on average a Pokemon looses 10% of its total CP per hour. Let’s take a look at the numbers. Below I compared a 100%IV Blissy maxed out at 3219 CP to my 91%IV Blissy at 2977. I too intentionally kept it under 3,000 because of how fast Mon’s decayed above 3,000CP.
Hour 0 maxed Blissy: 3,219 / my Blissy: 2,977 (CP difference = 242)
Hour 1 maxed Blissy: 2,897.1 / my Blissy: 2,679.3 (CP difference = 217)
Hour 2 maxed Blissy: 2,575.2 / my Blissy: 2,381.6 (CP difference = 193.6)
Hour 3 maxed Blissy: 2,253.3 / my Blissy: 2,083.9 (CP difference = 169.4)
Hour 4 maxed Blissy: 1,931.4 / my Blissy: 1,786.2 (CP difference = 145.2)
Hour 5 maxed Blissy: 1,609.5 / my Blissy: 1,488.5 (CP difference = 121)
Hour 6 maxed Blissy: 1,287.6 / my Blissy: 1,190.8 (CP difference = 96.8)
Hour 7 maxed Blissy: 965.7 / my Blissy: 893.1 (CP difference = 72.6)
Hour 8 maxed Blissy: 643.8 / my Blissy: 595.4 (CP difference = 48.4)
Hour 9 maxed Blissy: 321.9 / my Blissy 297.7 (CP difference = 24.2)
Hour 10 maxed Blissy: 0 / my Blissy 0 (CP difference = 0)
After both Blissy’s have been in the gym for an hour they will be at 90% of their full combat power if the gym goes uncontested and no berries fed. After two hours they will be at 80% of their full combat power, after three hours 70%, etc, etc. I’ll be powering mine up as soon as I walk another 50km even though I know that after about two hours in a gym its easy enough for a competitive trainer to take down. Hope this helps you decide how to use your stardust.