In terms of the JWST instruments? Not actually that long, maybe 15 years? JWST was already on the "drawing board" when Hubble was launched. The decades between JWST & Hubble were mostly working out the proving of the engineering origami i.e. folding it up into a rocket fairing, surviving the launch vibration of the solid rocket boosters, then unfurling successfully after parting company from the launch vehicle. The mirror on JWST gives a huge advantage in speed of observation, so tens of days for Hubble to collect light is now half a day & the things it can resolve are a fair bit better (witness the observations of planets in the solar system for example, or the Ultra Deep Field).

But mostly it operates in a different part of the spectrum, which is a game changer when comparing what JWST can reveal when looking at the same area of the sky as Hubble did.