Having attempted some research on this because I am curious and do want to know myself, I gather this:
It depends. And bleeding may not happen at all?
Where are you, who are your teachers and influences? And what is the nature of the fever, the agues...
I gather there was, among Learned Physicians, at least a vague sort of idea that fevers could be caused by a lot of different things, and a one-size-fits-all approach was inappropriate, though very commonly administered by the Unlearned.
A scolding polemic was published in 1728 that cautions against people employing bleeding unjudiciously, without taking into account the nature of the disease, the patient, the place, the time of year, etc. For a "bilious fever," if that's what's going on your story, bloodletting may not be indicated at all, in your doctor's POV--this book notes that "Dr. Friend takes notice of Alexander Tralli and his Practice in Bilious Fevers, that he used Purging and not Bleeding" and "this Method when pursued judiciously is attended with surprising Success," and that "Galen well observes that one Method of bringing this Distemper to a Crisis is by a Looseness. It is these sorts of Fevers Fernelius speaks of when he says, if they arise from Humours collected in and stagnating in the Stomach, or concave part of the Liver, they cannot be cured by Bleeding."
But then,
a dude named Cockburn in 1706 would disagree, saying that bleeding is really the best thing for a fever, since you can control how much you're letting out, whereas if you make a patient disgorge everything in their bodies by mouth and butt and urethra and so forth, you're just sort of giving them a poison and waiting it out, with no way of stopping it. (I'm paraphrasing here.)
And Benjamin Rush, in 1793, facing a "bilious fever," ultimately settled on copious bloodletting as the most important part of therapy...
Ah, anyway. So. Timing is hard to figure out...the 1728 book brings up some instances where Hippocrates "saved" patients by delaying bleeding for days because he knew the nature of the patient and the disease well enough to know when indicated, but doesn't get too far into what those indications are, as far as I can tell. There does seem to be an implication of sort of watching to see what happens first. He suggests that doctors take their lead on how to expel bad stuff from the patient based on what the disease is doing -- if it's causing vomiting, pursue vomiting rather than bleeding; if it's causing sweating, let them sweat; but if it seems localized in a place where it does not have a way of getting out, then you bleed...I don't know. This book is hard on my eyes; you might find benefit from a closer look at it though...
Benjamin Rush is a bit beyond your timeframe and this is for a very specific illness that may not apply to your situation...but he did write at least one book with a lot of numbered points that are easy to navigate
He wrote a lot about his experiences with attempting to treat the yellow fever epidemic of Philadelphia in 1793.
In this book, in the section "Of Bloodletting," he outlines what led him to think bloodletting was indicated, and he also says, if you keep reading through, that bleeding on the very first day of the "attack" was critical, or otherwise as soon as possible. He also specifies "I preferred bleeding in the exacerbation of the fever. The remedy here was applied when the disease was in its greatest force. A single paroxysm was like a hidden squall to the system, and unless abated by bleeding, or purging, produced universal disorganization. I preferred the former to the latter remedy in cases of great danger, because it was more speedyy, and more certain in its operation." But also:
"I bled in Several instances in the remission of the fever, where the pulse was tense or chorded. It lessened the violence of the Succeeding paroxysm."
However, I do think these remedies are things he came to by trying stuff out with sufferers of this particular disease; I don't think that this would have been a general recommendation for any random fever.
Bleeding might also be a first-shot remedy in cases where there is obvious localized inflammation...or where the patient requests it. George Washington died of some sort of acute inflammatory illness in his throat that left him barely able to speak or swallow, and over the course of a morning/early afternoon he was bled four times. (Also given purgatives, local balms, etc)
You might want to have a look at Hippocrates and Ibn Sina -- that's what I would do next myself, probably; but I'm out of time for internet rabbit holes at the moment
There are also probably a bunch of other medical texts written around the timeframe that address it...uh...hm. Well! An interesting ride, really. Looks rather complex. I do think you could probably get away with "whatever" as I think different doctors had different opinions on how to do things, as long as you justify it somehow...
(ETA: I do like doing research on medical history and could come back to this if you have more specifics that help narrow down the situation?)
