I know your question asks how many minutes shooting time that larger objectives give you, but I think the way to frame it is to look at it in terms of how much max magnification you plan to need to take a shot at dusk vs. your pupil size. Elkguide posted above that his effective 4.2MM exit pupil works at 10x with his Swaros. If feel you'll need 12x or 15x to take longer shots, then read on... I'm not an eye doc, but my understanding is:
Exit pupil size of your optics is important because on average, the human eye pupil opens to a max average diameter of approx. 5MM when your eye is trying to adapt to darkening environments (varies from person to person). Younger folks perhaps 4-9mm max in total darkness, but the older you are, the smaller your max pupil and this is partially why older folks often don't see as well at night. Therefore, in order to maximize the actual potential amount of light transmitted to your pupil and then your retina, your optics need to gather and deliver as much light as possible. Optics should optimally deliver an exit pupil of light matched to your eye for best dark vision, most folks will be about 5mm. If your variable magnification optics can deliver 5.6 mm of light at 10x (ie 56mm objective divided by 10 power), then most folks should be able to zoom in further until the exit pupil is about 5mm or a little smaller and still be able to see well in a very dark environment. The smaller the exit pupil of your optics, the less light transmitted to your eye and the more the image will degrade.
The above poster, elkguide, uses 10x Swaro binoculars w/42mm objectives, this delivers 4.2mm exit pupil and will work well during most archery hunting/scouting conditions since the eye is not fully dilated because it's not pitch black yet. But, if rifle hunting and wanting 15x magnification to take a longer shot at the darker side of dusk, then a larger 50 to 56mm objective will deliver closer to that exit pupil diameter. The trade off of course is that larger objectives and better glass cost more, weigh more and take more space. You'd better plan for at least a 4mm exit pupil at your magnification requirement in order to be able to shoot at the darker side of dusk.
Unfortunately, "trying out" or "testing" optics in a store is useless. Can you afford to buy two scopes at one time? If so, perhaps you could go to a store w/a good return policy, buy two scopes you're interested in, bring them to a wide open area at dusk, do not mount them, If you can, set up and scope known targets at long range as the sun sets. Compare the scopes and keep the better scope and return the lesser scope, buy another, compare it the next evening to the last winner and keep the new winner. Take notes of the ranges and zooms they worked at and at what distances they were acceptable. Run through this process with several scopes and you will reach an answer that satisfies you rather than trying to determine it from 3rd parties who probably have different eyes and needs than you may. When you're done w/your testing, buy the scope that best intersects your budget vs. needs and return all others.
Best of luck!
JL
Understood and helpful. I am trying to figure out if that extra length and weight of a 50mm objective vs the 42mm is worth the extra time it allows. 
