I'm curious as to how convoluted or detailed your adventures are in that case? What sort of games are you running? How much is player driven and how much is RP.
Same goes for anyone where prep time is less than play time.
I'll admit I don't run heroic fantasy very often, mainly Call of Cthulhu or other games with multiple NPCs, plots to foil and clues to follow, and I would say prep is several hours per hour of play although that does include prop making.
Focusing my answer on what I've been running mostly regularly for the longest: in my scifi space game in Cypher System, they are highly player driven sessions where I've developed enough details about the NPCs in order to create a model for their individual reacitons, complications, etc., prepped or ad hoc as the case may be. There is also a long-running narrative in the game, so it's not like I'm recreating a new set of NPCs and situations for each session --- each session more or less flows from the previous. Though, even when a session is a bit more "monster of the week" in nature, the prep work is pretty foundational.
Each NPC gets a name, a couple things they are good at, what they're currently up to, and why they are doing the thing they're doing. Places, more or less the same deal: name (or what people call it, anyway), a couple standout features or themes, what's there, and why the things are there (or why things are not there). Specific features (personality, nature of the flora and fauna, etc.) I'll just wing it.
From there the sessions are really just the table riffing off of each other. The party took a prisoner at one point and tried to wear him down over time to get info about the morally villainous megacorp that's a central point of the current plot. Did I plan out the depth to which said prisoner would resist interrogation, or be susceptible to a moral or empathic argument to be a turncoat? Absolutely not. I didn't even know if he'd be taken prisoner. I knew he was a megacorp merc sent to kill the former merc from the same megacorp in the party, and that he had his reasons for being mostly aligned with the megacorp. In the end he was held in the brig for an extended period of time, they wore him down to the point that they got fresher information about an angle against the megacorp, and stranded him on a livable world (hey, at least they didn't space him like the last guy).
Similarly, this hacker guy is mostly just about getting paid, and will take any ride out if it means he's a step closer to completing his deal or getting the next job started. Or this bioengineer is a believer in the cause, but their loyalty is more with the science than the megacorp, so they can be reasoned with if the party takes that tack. Or so on.
How much of that was I prepared for when I came up with the NPC (who in the specific example was really just a named combatant)? Name, profession, job, motivation. Everything beyond that is table RP and collaborative storytelling.