The best way to assure yourself would be to look at the specific bus information. Leaving Benidorm at 8 o'clöck a.m. isn't really hectic, so I wouldn't worry.
Although the buses are fairly cheap, if you are travelling a longer distance, don't forget to look at other types of transports such the trains (RENFE) or car rental.
Links to:
ALSA Buscompany - alsa.es
RENFE - Spanish Railway Comp. - renfe.es
CAR Rental Companies at the Airport of Alicante
