I don't make only pilots career, but rather squadron careers. After a pilot dies, I enlist a new one a few days after in the same squadron / escadrille / jasta / schlasta from spring 1916 to the end of the war. This way I go through all the planes.
Considering I have 1 scout and 1 bomber/reco career per nation running, I think it will take me at least a year to complete (I use auto/manual time advance and there are always no-fly days due to weather or visits to the hospital, plus the days of leave and aircraft repairs)
In my FFA 10 career, I had already a few days off due to lack of planes and pilots.
At the moment the open careers are RFC 32 and RFC 7 for British, Escadrille 69 for French (don't have two seaters units in 1916), FFA10 and Jasta 5 for Germany.
For Jasta 5 I am already in April 1917 because I moved this career from OFF to WOFF, the other ones have been selected randomly when enlisting the first pilot.