How long do egg yolks stay fresh once unshelled?

I own hens so my eggs are super-fresh. But my daughter likes to eat just the egg-whites, leaving the yolks in a cup in the fridge. Unbroken eggs are said to stay fresh a minimum of four weeks, but how long will the yolks stay good once out of the shell? One reason I ask is because I make mayonnaise out of the yolks, and I wonder how long that would stay fresh. It gets eaten up pretty quick and no-one's got sick from it yet!