Staying transformed no longer uses up his stamina

Inside the cavern, Vegeta smiled. His limits HAD been surpassed. No longer forced to rely on his anger to power up, he had stopped using the ascended state altogether, depending instead on his thoughts of Bulma and Trunks. This measure of serenity had allowed him to maintain his transformation constantly. His training hadn't been any easier...in fact, it had gotten harder and harder...but he had finally been able to make constant progress. Both his base and his transformed state had grown until he was nearly five times stronger than when he first fought Number 17.

- Bringer of Death Chapter 61