Napoleonic era Society

I read this somewhere (and now cannot remember where!) and would like to check whether or not it is correct. An officer promoted to (say) Captain in the field would not have paid for his commission (although he may have paid for an ensign or lieutenant's commission). I read that if he retired from the army, if he had been a captain for long enough he would get the price of a captain's commission even though he hadn't bought it. Is this correct? And if so, does anyone know how long he would have to be a captain to get it?