Bike light makers usually only specify run times for each mode -- bright, dim, flashing -- per battery charge. Usually flashing delivers the longest run time per charge. I haven't seen anything specifying the effect on LED life. That info is probably available by Googling around for LED components themselves rather than the finished products using LEDs.

This is one of the favorite bones of contention for gnawing on every bike related discussion.

I do what seems to work for me (i.e., reinforces my confirmation bias).

For white headlights:
Flashing in daylight.
At night, flashing in heavy traffic and near zones where drivers are usually distracted (parking lot exits, intersections).
Steady in most places at night, especially on the bike trails.
On the bike trails I usually try to use the dimmest practical steady beam and aim it downward to avoid blinding oncoming cyclists, joggers and walkers.

For red taillights:
Flashing/strobing, nearly always, day and night.

The only exception is in group rides when I either turn off my Cygolite Hotshot or set it to steady dim. Even the Hotshot 50 is too bright for group rides on any flashing setting. There's no way to dim the maximum output. The only way to do that is on the steady mode.

My hybrid/cruisers have dimmer red LED/reflector combos for group rides and most neighborhood rides. They're bright enough for that without blinding anyone, and the reflectors enhance visibility. These have better side visibility than the Hotshots and similar lights with intensely focused beams that are visible mainly from the rear.

I also wear helmet lights.:
A Vivo-Bike Illuminati on the front -- size of a Bic lighter, also available under other brand names via Amazon, for $12-$20.
And a Blackburn 2'Fer on the rear, usually in red flashing mode. Great, versatile little to-be-seen light, with steady and flashing white and red modes.
Both are lightweight, weigh almost nothing, last about the same time per charge per mode, and have good side visibility.

Helmet lights enhance our visibility to drivers. They're visible above car roof lines in traffic when our bike mounted lights are blocked. And it gives drivers two vertically oriented points for quickly estimating our orientation, distance and speed. I began combining bike and helmet mounted lights after observing this effect on other cyclists -- made it much easier to see them in cluttered urban settings, and to estimate their orientation.