When are ducks ready for the table?

Hi everyone
This is my first post on this very useful website - many thanks to the moderators for setting it up.
We are new to duck keeping and have 4 white ducks bought at a local market when about 4 weeks old. They are growing very fast and are now about 8 weeks old and we want to ask, is there an optimum time for killing them for the table. At about how many weeks old is an optimum time?
Many thanks for any advice.