The repairable OBJ (radar, ord, fuel, barracks, auto-ack) on a field will stay down 2 hours if the supply truck convoy is completely destroyed when it spawns every 15 minutes. The timing is not absolute either, I dont believe. The convoy spawns every 15 mins regardless of damage or not. So if the destruction of the OBJ took place at the 12 minute mark within the 15 minute spawn cycle time, then the timing is 12 minutes in favor of the field (convoy is only 3 min out w/ first set of repairs). If the destruction of the OBJ took place at the 4 minute mark, then the rapairs are 11 minutes out.
It is a roll of the dice as to which truck is which, but I believe each truck has different cargo reprenting the different repairable OBJ on a field. How many trucks are there? I forgot. If there are 5 trucks, then it makes sense that each truck represents each of the repairable items on a field. Remember that hangers and manned guns are NOT repairbale, they "rebuild" every 15 minutes regardless.
When you destory a truck, regardles of where it is at on the route, it does NOT reappear. A new and complete convoy will respawn. It is a 15 minute cycle.
This is one of those things only HTC really knows the answer to.