How far should the well for a temperature gauge extend into a 3/4" pipe?
I guess it depends on what you consider significant, a half inch pipe is pretty small and depending on the rate of flow I imagine it could be an issue.
But you can have your cake and eat it too - you can install a gauge without the well.  There are external sensors made to attach to the exterior of the pipe like this thermistor

This one is $12 and much less work to install.