I have never needed to hold a spot that long (15 years in home care). The one time I did it was for a sibling of a current great family and was for about 2 mos longer than the spot opening. But other than that , with siblings being born or new families, it just always worked out within a week or 2 of openings.
But....I had 2 infant spots opening up this summer. I had a family pursuing me very strongly, so I finally interviewed with them. She was barely pregnant! But I decided if they wanted the spot that bad, I would add a holding fee policy to my book. It is minimal, only a 1/4 of the weekly rate (so the same essentially as the other poster who said a weeks pay each month). I could have asked for more and I think they would have gone for it, but I am happy with this. The baby should not start until Labor Day week, which works for me in many ways (I need some time off to get my daughter off to college in late August, plus I start online college classes for a further Child Dev degree in the fall, so it gives me a week or 2 to get that started and to adjust, etc).
You need to decide what its worth to you, how heavy your local market it with interview potentials (mine had a ton), and if you really like this family and you mesh well (that was a big reason I decided to do do the holding fee...I had to have something $$ to hang onto it, and to see how truly committed this family was to me).
Good luck!!