On this page under the section "the sieving clients", there is a rough giude to speed that reads as follows..
Hope that helps.As a rough guide, a 100G range will take 3 days on an AMD XP2100+, 6 days on a P3-850 and 15 days on a P2-400. Again, as a rough guide, if that 100G range is around p=200T, expect it to yield about 10 factors.