In the US PNW coast area Douglas Fir trees are harvested for lumber within about 30 years, plus or minus. Maybe the person you were talking to was considering the harvest of the tree to be the moment when the CO2 is "reclaimed"?
Wrt to when the tree pays off the carbon footprint generated by raising and planting the seedling, I guess it's less than three years.
Fun fact: Douglas Fir reach peak carbon fixation rate at about 120 years.