After entering the vampire world, Rudy spent 10 minutes with Rias when he removed the shackles from her body. Then, it took him 2 minutes to reach the palace and three minutes until he met Virgil.
Viral took Rudy to the arena that was five minutes' distance from the palace, and the next five minutes were wasted talking with Virgil. And then five more minutes into the battle and coming back to the valley.
Thirty minutes had passed, and Rudy had only thirty minutes left. After that, he spent the next five minutes trying out the new ability.
Rudy had less than thirty minutes left to wake Rias up, and it was nerve-wracking for him. Still, he spent the next 20 minutes kissing Jane.
Of course, kissing her wasn't his main objective. He only did that so he could fast forward the time to spell instead of Rias' body.
