What do bloodsucking animals actually feed on?
Lets find out how much calories are there in the blood. I am using this site for main reference, but will also add more references where needed. But before that, we should know that one donation of blood is about 1 pint (or 450 ml) blood1. But here, I calculate with volume 500 ml.


First calculate calories for plasma. Blood is denser than water by six percent2, so multiply that 500 ml donation times 1.06 to get to grams (because calorie content will be calculated from grams). Multiply that by 55/100 for men or 60/100 for women to get to grams of plasma. Now, multiply that by .07, because plasma is only 7% protein3, to get to grams of protein. Finally, multiply that result by four to get protein calories. Here, considering 55/100, I get 81.62 calories.
Now for sugar in plasma. Normal fasting blood sugar is around 0.09 grams per 100 ml of plasma (more familiarly expressed as 90 mg/dl4). Multiply the amount in grams time 2.75 for men or 3.00 for women, i.e., equal to how many hundreds of mls of plasma there are in 500 ml of blood, and then times four to get to calories. Result should be 1 calorie.
For plasma calories from fat, use 0.5 grams per 100 ml5 of plasma, times hundreds of mls of plasma, times nine - for calories from grams of fat - to get to calories.  Result is roughly 12 calories.
Next comes calories from red blood cells. RBCs are mostly water and protein, the protein almost entirely hemoglobin. Hemoglobin does not conform to the rule of four calories per gram because the heme portion of the molecule is not metabolized for energy. Use average hemoglobin values of 160 g/L for men or 140 g/L for women6, multiplied by 0.5 to get to the blood donation volume of 500 ml, multiplied by 0.96 to correct for using only the non-caloric heme portion of the hemoglobin molecules, then multiplied by four to get to calories. Hemoglobin is only 90% of the dry weight of RBCs7. Multiply one-tenth the amount of the hemoglobin weight in 500 ml times six (see below for why six) and add that to red blood cell calories. I get 353.28 here.
Finally, estimate calories for the other types of cells. Of the 500 ml blood donation, roughly one percent will be white blood cells and platelets8. Figure five grams, times one-third to get to dry weight, times six, to get to an estimated 10 calories. The "times six" is because some of the dry weight is protein or carbohydrates, which both have four calories per gram, and some is fat, which is 9 calories per gram.


Add everything up. The total should be approximately 425 calories for women and 460 for men in 500 ml blood i.e. even 10 ml of a man's blood contains about 9.2 kcal (there's no definite answer about how much blood a leech drinks at a time). Now, as you say in your question that a leech requires about 0.714 kcal, it means that a leech can survive for almost 13 days by sucking just 10 ml of a man's blood (if they have to metabolize everything present in blood (except, of course, water)), which justifies your statement.