I am no doctor, but intuitively, you think recovery time would vary on factors such as the extent of the damage, the location and type of tear, etc. So let's just hope for the best and mentally prepare for the worst.
And even once the ACL is "healed", how long will it take him to get back into NFL football shape? And to feel comfortable enough mentally with his twice injured knee to go full tilt in a game? Or.... do those things ever happen? Is he ever the same? I would not bet against OBJ.
I would re-sign him and have a realistic hope of him essentially being a playoff contributor. Kupp/Woods/Jefferson is a mighty fine trio so OBJ should take his time and not return until 100%. I would do a 2 year deal, if he is interested and the two sides can make the money work.