Most filaments you can leave in the extruder indefinitely without any ill effects.

There are some filaments that need to be stored away from moisture, particularly Nylon, because they absorb moisture from the air and don't print well if they contain a lot of absorbed moisture. However, this isn't an inherent issue with having the filaments in the extruder (if you had some setup that protected the filament from moisture while in the extruder, that would be fine as well - but in most cases it is more practical to store such filament in an airtight box).

Most commodity filaments (ABS/PLA/PETG) don't suffer from this as much (PLA supposedly also absorbs moisture but I haven't noticed this to be a problem, perhaps it depends on the conditions of the room in which your printer is kept) so they're fine to leave in the extruder.

If you "planing" to leave the filament there for months, then it would be a good idea to store it away, but for most filaments it is not a big deal to be stay loaded... provided that the humidity is not high (e.g. a shower or kitchen next to it would be not so good).

My experiences with "moistured" filament is that they get brittle, but only breaks when I don't print something. And it is a pain in the a** to get out the last part of the filament that broke right at the entrance... so store it away is always a good idea, if you are not too lazy ;)