I guess it depends on personal interpretation when looking at the word ''short time''. It could be 2-5 minutes of combat or even instantaneous like KP said. Me personally, I think Goku and Vegeta are able to and have been using their full power many times in the manga. There already clear signs of them losing power when doing so. Just look at Vegeta in chapter 20 when facing SS Goku Black after getting a Zenkai boost. He did much worse against Black before eating the senzu bean than he did after when all he's been doing beforehand was giving Black the beating of his life before he got a zenkai. Later in Chapter 23, Goku even says the reason Vegeta is switching between God and Blue was because Blue can't be used for a long period of time, which goes with the ''Blue at 100% can only be sustained for a short time'' statement. Even going back in the Champa arc, Goku went straight for a Kamehameha when he transformed into Blue to finish the fight quickly. So albeit KP's concept is interesting, it isn't really implied by the story. CC's theory on SSB's faults being the only reason Vegeta lost to Goku Black is half true and half wrong. Vegeta was indeed hindered by the forms strain, but he was outmatched in terms of power either way. But if it wasn't for his RoSaT training to switch between God and Blue, it's possible that even though he would be stronger than Goku Black because of the power increase from training, he would lose in the long run due to not being able to fight at full power while using SSB for a long period of time.
SSB Goku FP - 5,000
SSB Vegeta FP (Pre-RoSaT) - 5,000 and decreasing
SS Goku Black - 5,400
SSR Goku Black - 6,750
SSB Vegeta FP (Post-Training) - 7,500
Merged Zamasu - 10,000 (Low-ball, possibly higher)
''Completed'' SSB Goku FP - 10,000 (Or more)
Long read I know... don't bother if you don't want to