Cream liqueurs generally are recommended to be consumed within a year (e.g
baileys) but is does depend on the alcohol percentage and preservatives found
in it. An educated guess would say that an unopened bottle of Advocaat over 18
months old would be spoilt, whereas an open one last around 6 months.
But, (big but), the main indication of spoiling is a darkening of the liquid
or a scum forming on the top of the mixture (showing oxidisation) if it does
not have this chances are it may be OK - but for how much longer....?
Jay