Do you know the breed? or the sexes? usually mallard derived by about 4wks you start to hear quacking or this undertone rasp for drakes... You don't need to wait months nor till drake feathers occur and in some breed coloration of the feathers too indicate male vs female.
I will say in the effort of good practice this is a risk, you risk predators and most definitely not getting them back out. After that, food. food and more food.. did i mention food?? the way to a ducks heart is through their stomach if they learn you are that source they will follow you, trip you and heck knock on the front door(no, not kidding i have some who do) My Pekin will respond to a verbal command.. if i call Pekin they will come.. my scovies and buffs nope.. they'd think i was off the cogs.
I still feel i would aim for housing that you could lock them in safely for overnight ... many predators swim too you don't want to create a 'sitting duck' but again food will be the most helpful tool for getting ducks where you want them and to come to you.
As for laying sorry, usually around 6mths that said we are running later in the season, typically i would like to say my flock ceases in Oct and resumes in feb/mar but this summer has been hot & dry and my birds are not laying as much as typical for late summer.
I have 6 Ancona's and a really tall runner (he's got to be the coolest duck I've ever seen, I've named him Scout. :) ) Mostly hens, the runner is a drake and I have one drake Ancona.
Safe housing is the goal. I want to build a floating duck house, but if I can get them to come in at night I will also have a nice pen with lots of cover. I really do care about them, I want my ducks to be safe, but I also really need them on the lake for most of the day. I've done ducks in pens 24/7 before, and I've done ducks on the lake 24/7 before. Ducks in the pen all day make my yard a smelly mess, ducks out on the lake 24/7 do well for awhile but I eventually end up losing them. So I want to try something in the middle. If it doesn't work I more than likely won't be doing ducks or any other Foul again...at least not until I move somewhere more suitable.
Yeah, I figured it was getting a little late in the season...do you think supplemental lighting might work? I mean, I can wait, but it would be really cool to start getting eggs now.