It causes no harm, in & of itself.

The length of time you can leave a machine without reboot really depends on how the apps you are using behave. A set of well-behaved apps will nicely hand back memory when they no longer need it & leave it then in the hands of the OS, which can release it if another app needs it.
So long as this & other potential 'clogs' such as networking ports behave themselves, there's no real need to reboot.

If anything seems 'not quite right' then a reboot is always first thing to try.

A quick check round some of the machines here, 3 different OSes, Mac, Win, nix gives 1 day, 10 days, 18 days & 85 days. Each will probably have last been rebooted for some update or another.
Windows will probably need rebooting the most frequently because it issues OS updates more frequently than the others. The 1 day is Windows, set to auto-update, so it reboots itself when it wants to. It rarely has a human at it, so that's fine. It never sleeps & usually updates overnight, when no-one needs it.

Half the machines here never sleep. The record holder - unfortunately now gone - ran 10 years uninterrupted, never sleeping, only ever rebooting for OS updates.

It causes no harm, in & of itself.
(Power savings & battery life are outside the premise of the question)

The length of time you can leave a machine without reboot really depends on how the apps you are using behave. A set of well-behaved apps will nicely hand back memory when they no longer need it & leave it then in the hands of the OS, which can release it if another app needs it.
So long as this & other potential 'clogs' such as networking ports behave themselves, there's no real need to reboot.

If anything seems 'not quite right' then a reboot is always first thing to try.

A quick check round some of the machines here, 3 different OSes, (Mac, Win, nix) gives 1 day, 10 days, 18 days & 85 days. Each will probably have last been rebooted for some update or another.
Windows will probably need rebooting the most frequently because it issues OS updates more frequently than the others. The 1 day is Windows, set to auto-update, so it reboots itself when it wants to. It rarely has a human at it, so that's fine. It never sleeps & usually updates overnight, when no-one needs it.

Half the machines here never sleep. The record holder - unfortunately now gone - ran 10 years uninterrupted, never sleeping, only ever rebooting for OS updates.