I expect a response to mail messages within 48 business hours. While 5-7 days may not seem like a lot, if you let three, four, five days go by between every (or even some) correspondence you can quickly find yourself at three weeks without a whole lot of progress. Then if the arrangement doesn't work, how long is the PSM left to find someone else to fill this position? If you go through that song and dance three times you can easily find yourself up the creek.
Earlier this month I ran into a similar situation with a technician I am subcontracting to complete service. The first tech I contacted didn't respond for seven business days. By the time he did, I had already contacted someone else, heard back, and contracted them for the work. The work isn't until late January however I wanted to resolve the issue, respond positively to my client and move on to the next item on my list.
This isn't exactly the same as booking an SM, however the principle is indentical, sometimes the first to respond does get the job. Keep in mind whoever is doing the hiring also has many other tasks or responsibilities. And yes, the employer definitely has the upper hand, but that's the way it is.
