In typical average everyday use, not much at all if you get a high quality external SSD. Source: Me, with my M1 Mac mini using a Samsung T7 connected via USB.

In typical average everyday use, not much at all if you get a high quality external SSD. Source: Me, with my M1 Mac mini using a Samsung T7 connected via USB. Things I use frequently are on the 256GB internal SSD, such as Office 365 and Xcode. Huge things like games that aren't a huge deal if they take a few more seconds to load are offloaded to the external. The only inconvenience this setup has caused me is that I have to periodically uninstall old iOS simulators from Xcode to keep enough free space available for OS updates.