Call ducklings - when to go outside?

The babies are almost fully fledged - just some of their wing "finger" feathers are still sheathed. I don't want to shock them with a sudden change in temperature. They are in a brooder in my bathroom but I've taken away the heat lamp two days ago, although there is one over a second brooder with small chicks in it, also in the bathroom. So the room itself stays around high sixties (I keep my house cool). The outside temps here are low fifties during the day and mid to high forties at night... so far. I've got a smallish coop-ish sort of of facility for them, and LOTS of pine shavings.

There are four of them, and they're about six weeks old . I kept my Cayuga pair inside until they were 8 weeks old... but that was in the Spring. How much longer should I keep them inside?