Following the question asked here: Can the Flash run faster than light?, well of course they can (all except Jay Garrick as I've mentioned here).
But how long does it actually take for the Flash to reach his top speed? In other words, is his acceleration to light speed (or near light-speed) instantaneous? Is anything of that sort mentioned in the comics?
Open to answers from all credible and/or official sources on any Flash since almost all of them have reached near light speed at the least.
Well, its hard to say for certain unless you have a moment in the comics or movies that I can't think of, but I did come up with a way to answer this.
We have to start with
Assumption 1 - humans have a constant rate of speed production
Not total speed but rather the multiplier from '0' to Max (note that I am using meters per second). So, lets look at Usian Bolt:
And other sprinters:
In the first second of running Bolt 5 M/S, which is 42% of top speed. Lets call this the first interval.
Interval 2: 83%
Interval 3: 91%
Interval 4: 95-100%
Each of the other speed athletes reached max speed by the 4th interval.
So, given that human bio-mechanics are unaltered, Flash will maintain this same multiplier and will take 4 intervals of time (whatever time it may be) to reach top speed.
Assumption 2 - That we have the correct max speed of Flash
Top Speed Estimate 1 (fastest flash ever): Top speed is 1750000000000000000 miles/second given the math done by monster stomps calculations.
Top Speed Estimate 2 (baby flash): The speed of light: 186282 miles/second plus 1 mile per second (so that it is faster).
Timeframe: 1 second
This timeframe is required as you must move the distances above to have been measured.
Estimate 1 Answer: 1.4525e+18 in 1/10000000000000 of first interval he hit lightspeed. It is so fast that it is, for our terms, instant (near to 1/10000000000000 of 1 second).
Estimate 2 Answer: 4 intervals to reach max, would mean that no matter what happened he would take 4 seconds to reach light-speed (max).
Really the bottom line is that a human takes 4 intervals of a given distance and time to reach max. Assuming that holds true for flash is the only way my estimate works.
If you piecemeal past feats you can make it functionally instantaneous.
In Flash lore, speed is demarcated by the following barriers from slowest to fastest (necessary speed to break said barrier): Sound, Light, Time, Dimensional, & Speed Force.
By the mid-90s, in Mark Waid's run of The Flash (Volume 2) during the Dead Heat story arc (villain Savitar), it was established that the Flash family could- more or less at will- enter the Speed Force even indoors (specifically, while in the hallways of a castle infested by Savitar's "Speed Ninja"). Being able to leap to the fastest possible barrier on that short a runway (indoors), means being able to get to the lesser barriers even more quickly.
Even if in narrative time running in the Speed Force Dimension seems longer, it's supposed to be instantaneous. Even if it isn't, however, as the Speed Force Barrier is past the Time Barrier, Flash can leap into the Speed Force... go back in time, get up to speed, then reemerge from the Speed Force at light speed with "instantaneous" acceleration from a linear time perspective. These kinds of time tricks allow Flash to race himself.
Flash can time travel under his own power, proven many times (as above), but most casually, when Wally goes back to obtain the Cosmic Treadmill when they need it for a plan after its already been destroyed in the Flash Museum. If you can dictate when you return to the time stream, it makes less material difference how long it takes you to get up to speed!
In practice, for anyone but other speedsters, Flash's acceleration is basically instantaneous on the high end... and as slow as it needs to be for plot purposes.
