Erik Answer

Like so?

signals.Publish(_signals => _signals.Where(s => isBusy(s)).Publish(_busy =>
from s in _busy
        from n in _signals.Where(s => isNotBusy(s)).TakeUntil(endTime).Count().Where(n => n == 0).TakeUntil(_busy)
        select TurnIndicatorOn)))))))

--------B----N---N---------B---N------N----N---B---------------------------- _signals
--------B------------------B-------------------B---------------------------- _busy
        |----N----N---x    |---N------N----N--x|----------x                  _signals.Where(s => isNotBusy(s)).TakeUntil(endTime).TakeUntil(_busy)
         -------------2     ------------------3 ----------0                  _signals.Where(s => isNotBusy(s)).TakeUntil(endTime).Count()
----------------------------------------------------------On---------------