I free feed all my birds, ducklings should have feed/water 24/7 for a few weeks for sure... i think my last batch were 8-9wks when i started removing it at night, usually i don't do this but the stall was a mess and they were not able to get the space they needed.
My 5wk calls still have access 24/7.. almost all my adult birds have access to feed/water in the main barn, with the exception of one mature drake and the four 11wk olds, as i said these are all housed in stalls more difficult to provide in those. The main part of the barn is easier to maintain feed/water...
Anyways, they all free range(Muscovy) none are overweight... i fill feeders in the morning, the one in the barn is never empty by morning and i don't have it anywhere filled to capacity(25lbs), there are 9 ducks in that area.