Gameshape effects can last many games as poor minutes (as you would get from three starts) can cause you to drop multiple levels. So if your players drop from strong to average you have a week of disasters, then if they improve to respectable you have a second week of poor results, if anyone has too many or too few minutes (often happens with injuries or foulouts) then you can have guys underperforming for a third week. In my opinion it has to be a REALLY important game to risk two or three weeks of losses. I would rather throw one league game during the week, build enthusiasm and then have my starters for the cup.