[angvelem]

Удобнее использовать Now.

Код:

var
  isDone : Boolean;
  ApplicationStartTime : TDateTime;

procedure TForm1.ApplicationEvents1Idle(Sender: TObject; var Done: Boolean);
Var
  CurrentTime : TDateTime; 
begin
  CurrentTime := Now;
  if not isDone and (CurrentTime >= ApplicationStartTime) then
  begin
    isDone := True;
    ShowMessage('Другая форма');
  end;
end;

procedure TForm1.FormCreate(Sender: TObject); 
begin
  isDone := False;
  ApplicationStartTime := Now;
  ApplicationStartTime := incMinute(ApplicationStartTime, 5);
  Application.OnIdle := ApplicationEvents1Idle;
end;