How quickly does soy lecithin lose its effect?
The FDA defines a “Use-By” date as the last date recommended for the use of the product while at peak quality. The date has been determined by the manufacturer of the product. After the "Use-By" date the product loses potency and/or freshness.  "Use-by” dates usually refer to best quality and are not safety dates.

FDA Product Dating

The product is likely safe to use after this date, however, it will be less effective or fresh.  As these dates are set by the manufacturer and vary wildly by manufacturer.  Soy lecithin degrades very slowly as noted by SAJ14SAJ.  You likely have a good amount of time (up to 24 months from date of manufacture) before the product is degraded sufficiently to require recipe adjustments.