There is also a tradeoff where every read from a CCD introduces a fixed amount of noise (called read noise), so there is a cost to making extremely short exposu

There is also a tradeoff where every read from a CCD introduces a fixed amount of noise (called read noise), so there is a cost to making extremely short exposures. As a rule of thumb most individual exposures on a large telescope are ~20 minutes or so for an image where the plan is to stack many exposures. But sometimes different fields have unique constraints, and obviously JWST is a different beast than ground based telescopes.