In the "Future Trunk" arc of Dragon Ball Super, Vegito didnt last an hour fused because he had so much power he consumed all the energy to stay together. But now we've seen Kefura released a huge amount of power, probably close to Goku Ultra Instinct, and probably over Super Saiyan Blue Kaioken since we've seen her having an even match with Goku in that state and now she turned into super saiyan 2 meaning she multiplied that power by 2. Then why Kefura didnt defuse after releasing so much power?
You are comparing the power of Vegito Blue to Kefla. We know from the start that Goku is extremely fatigued so he wasn't at full power when he was using SSJB. Also Goku didn't have the power in him to use SSJB+Kaioken*20 which would've been sufficiently enough to defeat Kefla. Vegito SSJB Is a lot more power than SSJB + Kaioken*20 Goku and is on a completely another level compared to Kefla. Vegito SSJB was sufficiently strong enough to force a god of destruction to fight seriously based on Whis's comments where she said that Goku and Vegeta together could go toe to toe with Beerus. Kefla, on the other hand, is not at the same level.