Sadly there is no real way of knowing as everyone is different just like what caused their depression in the first place.
If someone has been on them short term because a particular issue in life has caused it, and that issue is no longer present then MAYBE the depression wont come back.
However if the depression came out of nowhere then MAYBE that person is in someway genetically programmed with depression and MAYBE it could come back.
There really are too many things to consider, so many things about depression that aren't yet known and so much that will never be known to give a definate answer