Ducklings grow really fast. Gradually raise the lamp, you'll get a feel for it. If they cluster underneath it, they're too cold, if they sty away from it, it's too hot. They do naturally cluster so that's not a sign they're too cold necessarily. My ducklings were off heat between three and four weeks and I switched to growers/ finishers (same pellets for both) at the same sort of stage - when you start giving them food several times a day! Mine are about six or seven weeks now nd in with the main flock by day. They're still smaller but I've started to do the odd double take between them and the adults so they'd be good eating soon.
H