My 713 Hertz certificate says "25% off a Hertz rental for up to eight (8) days on select car classes at participating corporate airport locations in the US & Europe"."Certificates are non-transferable and must be used for consecutive rental days and will be deemed fully used once a rental has begun". "...not to be combined with any other certs, promos, discounts, or special rates".
Othrer than that, I don't see anything specifically prohibiting you from renting the car for a longer period of time.