How long will it take for Henry to start feeling better?

Henry became poorly on Monday, he started being very lethargic and just not wanting to do much. We took him to the vet yesterday where he was diagnosed with swollen glands, a sore throat and a fever/a viral infection.

He was given an injection of convenia and meloxicam, this morning he was being sick so we took him again to make sure nothing was wrong and he was given another injection of cerenia to stop him from being sick.

I'm just wondering if anybody knows how long it will take for him to feel better? we hate seeing him like this!