You may be able to make a more exact estimate by working backwards from the Westminster lying-in-state, as reported in ITV news online -
"The Queen will also lie in state at Westminster Hall in central London.
Traditionally, lying in state begins five days after the day of death - known operationally as D+5.
The Queen died on Thursday, September 8, but because the announcement came relatively late - around 6.30pm - plans have moved by a day to allow time for the complex arrangements to be made.
The Queen's lying in state at Westminster Hall will therefore begin on Wednesday, September 14.
There will first be a ceremonial procession through London, taking in The Mall, Horse Guards, Whitehall, Parliament Square, and New Palace Yard.
After the coffin's arrival at Westminster Hall, the Archbishop of Canterbury will conduct a short service, which will be attended by members of the royal family.
Once that service ends, there will be the opportunity for politicians and dignitaries to pay their respects to the Queen.
Shortly afterwards, Westminster Hall will be opened to the public. Hundreds of thousands of people will file past the coffin on its catafalque and pay their respects, as they did for the Queen Mother's lying in state in 2002.
Senior royals are also expected to pay their own tribute, standing guard at some stage around the coffin in a tradition known as the Vigil of the Princes.
The Queen will lie in state for four clear days, until the morning of her funeral.