It happens with fair frequency and depends entirely on the reason for the underlying delay and often the station itself.

That is why UA, along with most carriers, requires that you be at the gate ready for an on time departure unless you are expressly advised otherwise.

Particularly at hub airports, aircraft are routinely substituted and this may mean that an expected 3-4 hour delay becomes 5-10 minutes. ATC delays sometimes go away and repairs thought to take a couple of hours sometimes take less.