Depends highly on how long the defenders held out, how many Romans were killed, how expensive the seige was, and how patient the emperor was.

One good example is when Aurelian subjugated Palmyra for the first time after their rebellion. No one was killed. Then Aurelian went back to the Rhine to deal with the Germans. He then got word that Palmyra was rebelling again, and when he got back for the second time, he killed everyone and leveled the city.

A good example of clemency is when Diocletian captured the rebellious city of Alexandria after a long and costly seige, and upon entering the city on horseback, he cried something to the extent of "I will kill until the blood rises to my horse's knees"

It was at that particular moment Diocletian's horse decided to sit down and rest, with his knees to the ground. Diocletian, seeing as this was an obvious sign from the gods, ordered all to be spared, and when he finally left the city, the citizens of Alexandria supposedly made a large bronze statue of Diocletian's horse