How often do vampires feed?

[Vintrastorm]

There's nothing in the SRD about how often, or for what purpose, vampires drink blood. Technically being friends with a vampire isn't that bad. Except for the part where they're evil.

[Diarmuid]

I'm pretty sure sure either Libris Mortis or BoVD gives mechanical rules for the various types of "hunger" most undead are afflicted with and the penalties for not meeting them.

[Silva Stormrage]

Libris mortis has those rules. A vampire loses all mobility after about 3d6 months and has to make a will save (DC 15) every three days or take wisdom damage if he doesn't drink blood and for life force (Inflicting negative levels) they must inflict one every day or make a dc 25 will save or take wisdom damage.

[BWR]

The book of bad Latin has vampires either losing mobility after 3d6 months if they don't eat or, optionally, having to make Will saves to avoid 2d4 Wisdom damage after more than 3 days without food. Loss of Wisdom means it grows less rational and Wisdom 0 means it's a ravening beast that must feed at all costs.

2e's "van Richten's Guide to Vampires" has that the average vampire must feed once per 24 hours. Failure to drink enough blood leads to a temporary loss of 1 HD per day of insufficient feeding until the vampire reaches 1 HD, at which it becomes a ravenous, near-mindless beast. Vampires can generally not starve to 'death' (a fact Strahd used to punish one of his enemies).
The exact emount varies depending on the age of the vampire and since blood drain dealt hp damage and not Con damage in 2e, you'd have to work something else out than 12 hp/age category (2 con points per age category, maybe).

[Fortinbro]

Complete Guide to Vampires gives vampires a blood pool which depletes when they don't feed and re-fills when they do. It can also be burned through as a resource for some feats. If you're running a campaign involving vampires, I highly recommend the book.