CCL: Computation Resource Requirement.

 CCL Users,
being new to the field of computational quantum mechanics, i have a basic question on the requirement of the resources for this type of computation. Can anybody cite any reference where i can get some relation between number of processors and time required with that of number of atoms in a system , basis set used, etc. In my system of Hydroxyapatite- polymer I have the following details:
 atom                 number
 Calcium              20
 Phosphorous      12
 Oxygen              54
 Hydrogen           10
 Carbon                 3
 Basis set -            6-31+G**
 Method -              HF (Hartree Fock)
can somebody give idea on processor-hour required. All my simulations are run on NCSA machines.
 All suggesions and information appreciated in advance.
 Pijush Ghosh.