I've only had one year's worth of experience with Gerberas, so no expert to be sure. Being somewhat fascinated by the plant, it seemed to me that as soon as I took down the stems of spent blooms, new buds started to form. The longer I left the spent blooms, the longer it took for new buds. The new bud stalks form in among the leaf axils at the crown - and grow to blooming size pretty rapidly - so cutting the old ones far down does no damage.