Well, it might be a strange "under the influence" forum request but if we see what's been already published:

09-11 X-Men Mutant Massacre
33-40 X-Men Inferno OHC
60-62 X-Tinction Agenda OHC
63-70 X-Men by Claremont/Lee Omnibus

from X-Factor #01 to Peter David's era would bé something like:

Avengers #263; Fantastic Four #286; and Classic X-Men #8 and #43 (Phoenix Rising)
X-Factor #1-8, 12-32, 41-59

This is roughly over 1100 pages.
I'm not an expert in mappings but, with the current thin paper, all these plus the 6 annuals could fit in just one single Omnibus?