It depends on how financially dependent you are filling that spot. A couple years ago I was interested in filling an immediate open spot in May, however an interested family wouldn't need it until September. Financially, I didn't absolutely need to fill it to make things work, so I charged them a holding fee of $100 per month (they dropped it off every month), and it was non-refundable, and does not go towards their fees when they start. This way, the spot is guaranteed to them but if they bail you're not completely out of $. Do up a "holding spot" contract that details the holding fee is no-refundable should they look elsewhere.