If your buying leads, and specifically internet based leads (internet based isn't the salient point here, the key is leads that are proactively in the market to buy your product or service), multiple contacts are needed to ensure you're given a fair chance at articulating your value proposition. In fact, there are two main components to reaching success with leads, speed and repetition.
We know speed is important and has been forever. Take the old-school yellow pages. Some companies would name themselves solely in order to place high/first in the phonebook. It's not an accident that 1Reason has the number "1" as the start instead of "One". Have the first chance to speak with someone is important because people are likely to go with the first one that meets their needs.
If you can't be first, at least try to be second. Lot's of good can come out of being in second place. Sure, you may not get the sale today, but if the other company fails to deliver, you're next in line (potentially).
According to LeadsCon (I don't have a relationship with them), the average time to contact various lead types are:
Mortgage 6.4 days with 44% no call
Insurance 2 days with 38% no call
Education 6 hours with 3% no call
Fortune 100 3.5 days with 33% no call
Seems that the Educational space wins this race. For myself during business hours, my average time is less than one hour, much less two days. Even when a lead comes in after hours, I still often will at least email right away (real email, not including the auto stuff).
LeadsCon says that the "Ultimate Contact Strategy" is as follows:
Three calls and an email
Call and Email
They say that by making the first call within the first minute of receiving the lead increases your conversion rate by almost 400%. Also, by making at least six calls, you can increase your conversion rate by almost 50%. Sending five emails increase by another ~50%
In other words, if you're not receiving the ROI that you want from your leads, maybe it's time to adjust your strategy.