Instantly, as i see you perform handseals or whatever. (Dunno how it's performed, it's not stated, and it's a CJ.)
If it requires no handseals, then it goes like this...
Although the Nagashi is made quickly and effectively, it requires that the user constantly adds a steady amount of chakra to it, to uphold it's destructive and protective power, therefor the user normally has complete and utter control over the Nagashi, this is proved since Sasuke Uchiha can extend it through objects without harming them. The Nagshi is therefor completely under my control, and requires my chakra to be upholded, your jutsu clearly states that. "the user focuses his lightning chakra throughout the air" Meaning that this is something similar to a bacteria taking over a cell, it brings a certain and constant amount of energy with it, and then attacks something that produces energy. In this scenario, it's simply a wave of chakra, that's going to take over an outer shell of chakra, but as it has been shown before regarding things that's being taken over by chakra, it isn't done instantly, and the chakra is also weakest in the area furthest away form the user, (see Jirobo's earth dome). Therefor your takeover will not be instant, and it will not be equally strong everywhere. If this theory is wrong, and this jutsu is actually capable of being equally strong all the way around, naturally the areas in the back will be the hardest and latest to be reached, and the amount of force needed to take over an A-ranked technique is pretty big. I should be able to recognice your chakra the second it creates unballance or distrupts the flow of chakra through my Nagashi, this itself isn't an escape, but this might be.
The second i feel the slightest disturbance in my constant flow around my Nagashi, i immediately stop all chakra additions to the Nagashi, and then i do several things at once.
1. I gather incredible amounts of chakra in my fists and all over my body, as if preparing for some giant jutsu.
2. I expel a wave of chakra in a 360 degrees angle all around me, not a constant wave, just a single hard and concentrated wave of chakra, which should be able to create an extreme chakra chaos in the Nagashi, or maybe even push it away.
3. I focus extreme amounts of wind based chakra into my fists, and expell them like giant hurricanes, (the Senpuuken) the wind will definetly blow all rests of the Nagashi away, and it will blow me several meters backwards in almost an instant.
If this theory holds, i should be able to do this before the Nagashi causes any serious damage, or damage at all.
Fuuton: Senpuuken (Wind Style: Hurricane Fist)
Type:Attack
Rank: B
Range:Short-Mid
Chakra Cost:20
Damage Points:40
Description: A vortex of wind shot out of the users fist wind he/she punches.