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.