How many years of life does the donor sacrifice in a partial liver transplant?

None. There should be no change in the partial liver donor's life expectancy. The remaining liver will be sufficient to allow for a full normal life.
Zero. Complication rate for a live liver donor depends on how much liver and what part of the liver the donor is donating. For adult donating to a small child the complications are rarer and more mild than adult donating to another adult. If the donor makes it through the surgery with only short term and mild complications (vast majority of cases), he/she should expect a normal life span afterwards.
Should not be much. Not known accurately because they have been done for only 20 years or so. After a living liver donor heals from surgery, the remaining liver will regenerate to replace some of the removed liver tissue. If healthy at the time of donation they should live a full life.