I personally don’t believe leaving mass early is ok unless there is an emergency. My family always liked to stay till the end of the last verse sung in a hymn, then we’d just wait around in the pews, pray, until the crowds cleared out of the church and the parking lot.
But, I do know that some people believe the recessional hymn is just a tack on to the end of the mass, so if they stay till the priest says, “The mass is ended…” then they’ve stayed through the entire mass. I’ve also noticed that people will stay during the recessional hymn, but only until the priest passes them along the aisle. You would then see the pews empty out from the front to the back. Some of the priests handle that “problem” by standing up at the sanctuary until about 2 verses of the hymn are sung, then they proceed to process out.