Sherrik, if you would like to pair them up then do it at least 3 weeks before you want to start to collect eggs. Anecdotally, duck sperm can last three weeks so if you don't separate them for that long then you can't be sure that the father is the drake that you have penned with the duck.
So, as a minimum, you'd want to pen each pair for four weeks - 3 weeks to be sure of who the father is and then 1 week to collect eggs. If you want to collect eggs again later you'd have to repeat this procedure, so how long you need to do it for depends on how many times you want to incubate eggs.
What are your plans - do you want to set eggs just the once or do you want to have multiple batches? If you want to have multiple batches then I'd separate them for the whole time you want to collect eggs, starting from 3 weeks before you want to collect the first egg and ending after you collect the last egg.
I hope you have another wonderful hatch rate this year and lots of beautiful babies!