I keep ours with a heat lamp until they are 3 - 4 weeks old. On nice days they go out on the lawn for ever extending periods of time (on wet / cold days they stay in and I cut them a turf to graze on, which they love).
I introduce water for paddling in the first few days but only for short periods and ensure that they have the lamp to dry under once they come out as they don't have the waterproofing that they would get if they were being brooded by mum.
When they stop sitting under the lamp on a regular basis then I conclude they don't need it anymore and are capable of regulating their own body temperature. It all depends on the weather, but at this time of year I would expect to have them under for at least 3 weeks.
Hope that helps...or maybe my goslings are just spoilt!!