When Did Kanan and Hera Conceive Jacen?

Jacen was at least a few years old when we meet him as Hera's co-pilot in the season's finale. This would mean he was conceived at least five-six years ago, roughly during the beginning of the Rebellion and not too far from the Battle of Scarif.

When did Kanan and Hera make a baby?

Jacen Syndulla was born to General Hera Syndulla of the Rebel Alliance on the planet Lothal in 0 BBY. His father was Jedi Knight Kanan Jarrus, who was killed in combat before Syndulla was born against the Galactic Empire while saving Hera and their crew shortly before Lothal's liberation.

Was Hera pregnant during Kanan's death?

This is again confirmed by Sabine's speech - as is Rex being on Endor - but Sabine also reveals that Hera was pregnant during the end of Rebels, giving birth to her and Kanan's son, Jacen sometime after the events of the finale.