I am not aware of a specific industry standard, but we do have clients that clean their boards with our defluxing chemistries right away after soldering or sometime within 24 hour time frame.

 here are also processes that we support where the chemistry is still capable of cleaning up to 72-hour old flux residues. There was also this one unique situation with one of our aerospace defense contractor clients where he stated:

“…The CCAs that I had showed you with the flux residue that had been on the CCAs with coating for almost a year came clean with 2 passes through the cleaner. Very impressive…”

So there are cases where you can clean up to 1-year old flux residue.