Miss Lydia,
This was an old post, but others might have the same question.
Many breeders recommend 4 weeks in a brooder day and night, but as you say, they can sure stink up a brooder! Cleaning even twice a day can't defeat the smell.
When do I put them outside? Depends on the outside temperature. I live in central Tennessee. It's in the 70s by mid April, but can get to the mid 50s at night. So, as long as the weather is in the mid to high 70s or better, I take them out of the brooder between the second and third weeks. I still bring them back to the brooder at night until the fourth week. Perhaps longer, if they haven't feathered out. Sunburn can be an issue if they are still molting and patchy. Once feathered, I put them into a covered 10'x10'x6 pen. I keep them segregated like that until they are 10-12 weeks old, then move them either to the orchard for grass and bug patrol or to our pond for algae duty.
If you have no more than five or six ducklings to brood, and you have a bathtub that you can spare full time for a few weeks, the tub as a brooder makes an easier clean up than a regular brooder. I use the hand sprayer attachment to hose down the tub and the wooden frame I made five times a day. If you run the exhaust fan, you can't smell the ducks from right outside the bathroom door. That's not the case when I have them in the chick brooder in our garage!
The frame I made for the tub is a really simple construction project. It's made from 1"x 2" lumber, and is covered with latex shelf liner that is as wide or wider than the frame and tub. The trick is to make it just wide enough to cover the side of the tub, but not so wide/long that it folds over.
To fit, measure your tub's length and width at midway, at the edge, and half way in between that on each side. Taper the slats that go between them. I measured and cut mine before assembling, but you could probably make a rectangle, then saw off the ends to match the shape of your tub. Most tubs are fairly oval. Mine was a bit wider at the drain end. Consequently, the frame looks slightly like a rose trellis.
The cross pieces make up the bottom of the frame, and should be wide enough to reach outermost slats. Three or four pieces on edge should be plenty. The top slats run end to end, and are laid flat. Lay them no more than one inch between slats. Don't worry about the gap; the shelf liner offers plenty of support. Screw the long slats to the bottom cross pieces. I recommend pre-drilling the holes so that the wood doesn't split. Once assembled, start, but do not fully insert four screws into four corners of the bottom of the frame as "legs" to let the water drain beneath it and to allow air to circulate.
I bought a plastic paint tray and framed it into the end near the drain.The edges rest neatly on the wood and liner, and support the tray level with the floor of their bathtub brooder. The ducklings don't have to climb up to get in. As the tray ramps down to the deep portion, it's still safe and easy for tiny ducklings to get back out of the water during the first week. I clean and refill it with 90 degree water at each cleaning.
It probably doesn't matter what color liner you use, but I put white shelf liner over the frame, and a brown square of liner in one corner at the high end of the tub. The ducklings seem to see the brown square as a nest. The warming light is aimed at at that spot to help keep it and them dry and warm. The thermometer is attached to the tub with clear packaging tape just above that spot.
Hope this helps!