There is no direct dialogue that addresses when the father “fell in love with long distance.” But some assumptions can be made. Tom must have been near employable age, as there is no mention of how the family got along financially otherwise. Laura’s education has progressed to high school and near-puberty, judging from her attraction to the Gentleman Caller; his memory of her is not fresh, however, and he is engaged, so let’s say five years have elapsed since graduation. So if we conjecture a departure time of five to ten years before the play’s time, we should be able to fit all the “factual” information. Tom’s departure, ostensibly to the Coast Guard, would also fit into the calendar. As a director or actor, these facts would determine how to play Tom's character -- patient long-suffering, etc. or brash and irresponsible.
The father's departure is not specifically stated. We simply learn that he left because he fell in love with long distances. However, we can assume it was not too long ago, as Tom is the sole supporter of the family.