What D takes in lieu of food. They contain dried blood plasma and nutrients. Dropping them into a cup of clear water produces blood that can be drunk Apparently this is a common source of sustenance for dhampirs.
An Ordinary dhampir would take one at each meal three times a day. D uses one on his fifth day in the village of Tepes, so it would seem he goes at least five days with out feeding comfortably. It's stated in Stuff of Dreams and the Reader's Guide that two capsules could sustain him for about a week, possible two at the longest. Although his constitution seems to steadily increase over time and can go without blood for extended periods, many times going through whole storied events longer than 5 days with no mention of feeding. He doesn't seem to need them to survive, just to slake his hunger, as there are Greater Nobility who have gone over 5,000 years with out drinking such as Count Braujou.
Capsules are very hard to come by unless one went to questionable doctors who go against the law or bought them off the black market. A jar filled with a thousand capsules would allow a dhampir to go a year without food.
Baron Byron Balazs'S carriage was stocked with dried plasma. Three times a day he dissolved it in water to fill his empty stomach. Synthesized by the Nobility’s science, it didn’t differ in the slightest from human blood in scent or taste or nutrients. But it was still no use. For the Nobility, feeding wasn't about ingesting nutrients. It was about sustaining the psyche.