I'm trying to get better at quick shooting, but mashing the aim button seems very inconsistent. Is there a specific frame I should be looking out for to get the timing correct or is the window pretty large to pull it off?

It's different for every weapon. For the handgun you're pretty much forced to mash the aim key in order to quickshoot. With magnum you dont have to mash all the time. Shoot the first shot like normal and start mashing when Leon is about to go back to his original aim stance.

Not sure about the grenade launcher.

Just mash and try to get a few succesful quickshots in a row and you will see how long it takes between every shot. Thats the same amount of time you have to mash, hold, mash.

Use Cheat Engine to lock your ammo count and practice that way.