How far?

Since last real deal correction in early 2016, there have been 3 notable dips, depending how you count. 

June 2016 Brexit drop, -6.1% high tick to low tick
Summer into election jitters, -5.0%
March 2017 dip off highs, -3.3%

A 5-6% correction targets 2340-65. 3.5% would be about 2405.

Assuming drop is for real and will match March at least, then the minimum target is SPX Q3P at 2402. If that breaks, there AugS1 in play for a few more weeks is 2377 and Q4S1 would be ideal at 2347, a totally normal pullback in line with moves historically common and matching previous drops in this rally cycle as well. 

Another way to visualize is to projecting by points like this. Results are nearly similar, giving a range of 2365-2380 or milder drop of 2410.