Having the eyelash expressions for some time you yourself will understand it. As soon as you want to return the original volume go for a refill. Usually it is made up to 3 weeks (on average 2-2.5 weeks). The retouch removes some of the old extended cilia (which are strongly grown, turned over, if any) and new ones are added to those of their own that have grown over this period.
Usually for retouch come with 40-60% of the eyelashes from the original extension. Each eyelash like hair has its own cycle of life and depending on how they are updated you need to make a refill.