We are dealing with indeterminate soybean varieties here in Iowa, and they will continue to accumulate biomass and set vegetative nodes onto what we call close to the R5 growth stage. A lot of times that will happen mid-August here in Iowa. After the R6 growth stage, when seeds are fully developed in the cavity of the pod, the yield has been determined and fungicide applications will not result in an increase in yield.
There are other restrictions as well. Some of the fungicides are only labeled to be sprayed until R5 growth stage. Also, some fungicides that have preharvest interval up to 30 days, so you will need be careful not to apply fungicide too close to the harvest.
Be sure to read labels and make sure you follow the label guidelines.
