In a battle with Kagetane, when Rentaro is about to die, he uses the AGV drugs to get superhuman habilities, heal himself, and finally be able to defeat Kagetane. Does this superhuman healing hability he got lasts or is it a one time thing?
It is once off, the effect doesn't last very long.
In the 1st light novel when Sumire gives him the drug it is stated that there are five vials, if it didn't wear off then there would have been no need to give him more than a single vial (or even inject him ahead of time, although there is the minor side effect of the drug turning users into Gastrea)
He instantly caught what she threw into his chest. There were five small syringes connected together like bells. Inside each was a red liquid, and a cap was on each needle. "That’s my going away present," she said. "It’s something I made while researching Gastrea. You know what I’m talking about when I say it’s the AGV test drug?"
Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (p. 132). Yen Press. Kindle Edition.
It was the AGV test drug, a drug Sumire created while studying Gastrea that made a human’s regenerative abilities go through the roof. This dramatic effect could even overcome the inhibiting effect of Varanium. If it weren’t for the side effect that twenty percent of the test subjects became Gastrea, Sumire would be so famous that her name would be in textbooks.
Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (p. 180). Yen Press. Kindle Edition.
Below is an extract from the 5th Light novel after Rentaro was injured, which clearly indicate that the quick healing capabilities had long ago worn off, otherwise he would have simply healed on his own several pages before
The gruesome sight almost made him groan. It was no longer bleeding, but the wound was still a blaze of pain.
"Hey, how did you stop the blood?"
"I stuck a hot frying pan against it."
Kanzaki, Shiden. Black Bullet, Vol. 5 (light novel): Rentaro Satomi, Fugitive (Kindle Locations 2128-2129). Yen Press. Kindle Edition.