A human can lose up to 6 litres of blood before any negative side effects take place. An average person drinks 2.7 litres of liquids a day on a healthy diet. It takes around 4-8 weeks to replace all the red blood cells lost in 2.7 litres of blood and 48 hours to replace the plasma, considering youre healthy. This means that effectively speaking, if youre in good health, a vampire goth gf could feed on you 2 times a month without you suffering any serious side effects. In fact, the russians ran newer blood through a elderly rat in an experiment and found that the rats cells started regenerating as if they were younger. So effectively, with my body making fresh new blood all the time, and her drinking fresh new blood all the time, we could, in theory, live forever.