If you go for an evaluation every 5 days (the shortest time interval possible) it only takes about 5000 experience earned in campaign in those 5 days to rank up. If you wait longer than 5 days the amount required increases as well. I've not checked in on the research on it lately, but I've just been going by a 1000xp per day rule of thumb and so far its worked for me. I ranked up Monday afternoon, I'll rank up again friday, and I'll earn enough xp to rank up once again next week before we lose beastman-controlled garlaige again. Then I'll take a week off probably until beastmen regain the region, and do ~10,000xp before trying to rank up once again.
You can check the BGWiki and the otherwiki for explanations, I'm sure they've got a lot more info than when I looked at it a while back and learned the 5k thing. I believe some theories were out there that its actually AN earned, not xp, that counts for ranks, which means it could technically take less than 5k xp since you sometimes cap xp early, etc, but I've got no confirmation one way or the other.
____________________________Pogle | Gilgamesh Taru | Windy Rank 10 | ZM 17 | CoP/TAU Complete
75 BLM|WAR|RDM|RNG|BST|SAM|NIN|MNK|SMN|DRG | 56 BRD
Social LS: Icefire, http://becd.net/ffxi/forums
100+2 Alchemy | 59.0 WW | 40 GS | 25.2 Smithing
I'll try being nicer if you try being smarter.