I agree with Momof4. I won't hold for longer than 3 months. With each month a charge a 2 week deposit and if longer than 1 month a $200 per month holding fee as well.
That said, there is nothing wrong with interviewing and keeping in touch with the family. You never know what happens tomorrow.
I had one family who Mom kept saying how much she loved ME list their home, sell it in 3 days, buy a new 1 hour away and not tell me. Then use the excuse when they told me a half hour before their daughter was due to arrive that she won't be coming any longer.
Cover all your bases always!