The answer to whether a hive will accept or reject a cell goes a bit deeper than simply how long they have been queenless. Other factors come into play as well. My experience is that a gentle honey flow is more conducive to cell acceptance and that nervous defensive bees are much more likely to reject a cell as well, particularly if there are some robbers sniffing around. The very worst cases of cell acceptance I have experienced are invariably in just such a hostile environment. If it were as simple as the length of time then one would have by far the most success placing cells in hopelessly queenless hives and anyone who has tried that knows it is rarely successful.