When some people have the 3 month flare up they go back to the intro diet for a few days and then re-add foods as per the stages chart https://www.wellbees.com/scd-stages
The length will depend on severity of regression, if using medications, what foods are eaten - it is an individual thing and varies according to each persons symptoms. Generally, symptoms improve faster when people use gentler, easier to digest foods for a while while symptoms are at their worst.