Towards the end of the first.
Before that, Tuvok had the two-and-a-half Starfleet pips (Lt Cmdr), Tom Paris had two Starfleet pips (Lt), and B'elanna Torres had something that varied from two-halves (??) through one-and-a-half (Lt, jg) through two (Lt) Maquis slashes ("half" meaning black). After that episode, things gradually settled at Tuvok wearing two (Lt), Paris wearing one-and-a-half (Lt, jg) and Torres wearing one-and-a-half (Lt, jg). The change didn't happen at once after "Prime Factors", nor to all the players at the same time. Tuvok's pips changed between scenes in "Cathexis" at the very end of the first season (or actually changed back and forth, as the scenes were filmed in an order different from the one in which they were shown).