Go look at the categories that bomber rank depend on. You will notice, NONE are time dependent. In fact, to score high on three of them (Damage/death, damage/sortie, hit%), one only needs one or two "perfect" runs to be in at or near the top rankings of that category.
Damage points and field captures will require a little bit of time, but not too much if you are always successful.
The ranking system was deliberately designed so that those with relatively little time to play could still "compete" for rank with those have no life beyond the game. For example, this is why we have K/D and K/T ratios, not total kills, as a part of the ranking system.
<> I saw Lusche's post (who got in while I was typing) and went to the score page. I see now what you are referring to. What I said above is still true generally, but Lusche is correct in this specific instance.