I think every agency recruiter needs to spend some time on cold calling. I try to cold call at least an hour during the morning. It really depends on how many contract I have at the time. If we don't prospect daily, contracts will dry up fast.
Here is some data from Bullhorn's 2011 Trends Survey regarding how much time is spent on client acquisition. You can check out the whole report here with insight on most popular tactics,etc. We just finished the 2012 survey which shows a slight dip in time spent on new client acquisition.