It depends on your specific need - some carers are easier to find than others. So to make the most of the site we recommend that you send 6-8 messages if you just need a babysitter or 10-20 messages for a nanny or afterschool carer.
Expect at least 50% replies and allow a 7 day turnaround, although it is likely you will gain a good number of replies within days. If your first round of emails and interviews don't introduce you to the right person, simply send out a second round the following week - there will be more people to choose from and you are likely to succeed within that week. For very unique requests (e.g., unusual hours), you may need to allow an extra week or two. You may also need to consider varying your expectations (perhaps using 2 carers for split shifts before and afterschool). Make the site work for you!