(Sub gravi is when a sin is committed to achieve an end. A levi gravi is a sin that is committed with no goal in mind.)
Each venial sin that one commits adds to one's time in purgatory. A venial sin can be left unconfessed. Venial sins remain venial no matter how many one commits; they cannot "add up" to collectively constitute a mortal sin.
See Jeremiah 7:26, Lamentations 4:6, Ezekiel 16:44-58, 1 John 5:16-17, Matthew 11:22, John 19:11.
"The Venial Sin" is a short story by Honoré de Balzac.