our company default provides 40 Gig for C-drive.
that is supposed to be sufficient for:
A-) win2k8R2
B-) additional software for each server (backup client, antivir, mgmt, ....)
C-) generic software , specific for it's use: application, office, IIS, ....
D-) "disk full" trheshold is 80%"; so A-C only ever use up to 32 Gbyte or below. Including page/hiber*.sys
seldom I receive complaints....
() DATA is stored on D:, if necessary...
() servers who need more, can get more, but 80% has enough with default