I saw a substantial initial Allied bombing effort that lasted all of 24 hrs, since abandoned, because it was driven by a single squad for a limited time with no one to carry the torch where they left off. You will not see a sustained effort because the knowledgeable Allied Pilot's Association is down in numbers, quite like, I guess, its Axis counterpart. You do not have a large pilot roster to work with. You cannot run RDP missions with greentags or F2P customers.
I and a few with me have been flying long hours of RDP intercepts, alas after the bulk of damage had been done. It has been by no means a sustained effort, particularly as no-one in general is actively watching radar blips in the wayward boonies where sneaky Heinkels like to lurk. If you don't get airborne (in tier 0) with at least 5 squares worth of clawing to altitude, you might as well forget it. I'm not complaining mind you, t'is all fair and good. I got my scalps and saved (some) factories for hours on end, but I'm not cloned.
WRT times, longer rebuild timers will likely lead to LESS bombers in the air, since intercepts aren't really happening.
To rejuvenate the bombing game, you need several things, apart from a bigger (capable) pilot population:
1. More, and more diverse, targets.
2. A fast German bomber with smaller payload (Do-17, Ju-88) and a slow Allied bomber with bigger payload (Wellington, Lancaster).
3. A better radar system that is limited in detection by what territory you own, with individual destructible structures.
4. A payoff that motivates the bomber pilot to set forth without having catastrophic results for the game world at large, as currently. I have no immediate suggestions here, but, as it only takes 3-5 bombers to completely wreck the game for everyone, it seems that the individual contribution should be considerably lowered. If you have more targets, downtime can be drastic. With few targets, it's counter-productive.
5. Points for supporting missions. Factory patrol earns you nothing unless you score a kill after two hours, when you get 12 points. Yay, NOT. If you escort bombers but fail to score, you get nothing but satisfaction and praise (which, arguably, is worth more). The air scoring system needs an overhaul pronto. Adjusting points way up really cannot involve much work, no?