Usually first thing in the morning when there is a delay to inbounds and few departures and occasioinally at other times when there are delays. That is on westerly operations. On easterly operations aircraft may land on the departure runway at the discretion of ATC but it doesn't happen often due to traffic.
As I understand it, use of both runways for landing between 0600 and 0700 happens regardless of whether Heathrow is on easterlies or westerlies.