If we take St Paul literally, then we are not allowed to cling to our anger for even a day (cf. Eph 4.26). I would like to make a comment, however, that many people are so embittered and furious when they are in a state of anger, that they not only cling to their anger for a day, but drag it on for weeks. I am at a loss for words to explain those who do not even vent their anger in speech but erect a barrier of sullen silence around them and distill the bitter poison of their hearts until it finally destroys them. They could not have understood how important it is to avoid anger, not merely externally, but even in our thoughts, because it darkens our intellect with bitterness and cuts it off from the radiance of spiritual understanding and discernment by depriving it of the indwelling of the Holy Spirit.
-St. John Cassian