Most of my contacts are by email since I don't put a phone number on my ads as it is too hectic to talk to parents in the midst of playtime and/or you end up playing phone tag.
Often I will let it go up to two weeks or whatever weekend is closest and then on the Sat send out emails to all those that have interviewed to ask them how their search is going and did they want me to keep them on my list of potential families or something to that effect. Often by Monday I get back at least a few replies saying they found care or some will let me know what they are looking for as in we have decided we want a French environment, or want someone closer to work, or with less children or more whatever but at least that gives me some feedback. There isn't anything I can do about their reasons but at least I know. If a family has emailed me for information but not gotten back to me I will send them emails saying I am setting up my interview times for the next week and did they want one.
If it is any help I interviewed 27 families for my one space last summer and it took me almost 2 months to fill it. Lost a lot of families because of my lack of French which they should have asked at the email stage. I do put it in some of my ads now that this is an English program. The family I did take had done 14 interviews before coming to me. The more people out there advertising the more parents will go see them all and that takes time. If the start date was more than a month away assume they are in the look at them all stage.