I will answer the question you asked, then the one I think you meant.
A parish priest is allowed to leave the parish any time he doesn’t have duties to fulfill… Fr. Joe even posted a photo of himself fishing in the “water cooler” forum, and I assume he was off of parish property at the time.
I think what you
meant to ask was “Can someone leave a convent/monastery once they have entered?”
In a monastery or convent, there are generally several steps to the process, sort of like dating, getting engaged, getting married. Each level wears a different habit to recognize the differences in their status and the process takes several years. Just as you don’t jump into a marriage committment, you don’t jump into religious life or ordination.
The postulant makes a committment to give the monastery a serious try (i.e. no quitting after just six weeks!). Then she takes temporary vows, and becomes a novice. There may be another set of temporary vows, but, after a period of years of discernment, prayer, and living in the community, she will make permanent vows.
The permanent vows are binding and, well, permanent. Just as with a marriage, a person changing their mind at this point would have to go through some proceedings to have their status changed. The postulant or novice can decide (or may be told by the head of the monastery) that they do NOT have a vocation to the religious life and go back to being a regular lay person with very little fuss.
If a priest is not also a member of a religious order, he doesn’t normally go through quite the same process. Once he is ordained a deacon, he may no longer marry (although, conversely, married men may become deacons). Again, ordination is very closely tied to marriage, except the priest is, in a sense, marrying the Church. Thus, he is not free to marry anyone else. If a priest wants to leave his ministry and return to being a lay person, he much go through a process called laicization. A bishop has further responsibilities by nature of his position, which would complicate his departure, but, as a priest, he would also have to be laicized.
for more info, try
newadvent.org and search their encyclopedia for postulant, novice, final vows, ordination, and laicization.
