How much blood can leeches suck at a time?

A grown leech can consume between 20 and 50ml of blood until he is full. A leech can eat only once in a year to be able to live. This does not mean however that they only eat once in their lifespan.

Some leeches live only one year; some several years.

I would say that a leech consumes about 70-250ml of blood in its lifespan.