According to a Harvard Business Review article, the value is roughly seven times better or worse, depending on how quickly you follow up. From the article:
Firms that tried to contact potential customers within an hour of receiving a query were nearly seven times as likely to qualify the lead (which we defined as having a meaningful conversation with a key decision maker) as those that tried to contact the customer even an hour later—and more than 60 times as likely as companies that waited 24 hours or longer.
For lead generation, I would argue that it's worth spending *most* of an AdWords budget during sales staff hours, because these leads will ultimately be several times more likely to qualify and move to the next step.
It's common to pay 50% more per lead during business hours, causing many AdWords advertisers to focus more on off-hours (evenings, nights, weekends). I don't know about you, but I'd gladly pay 1.5x more for a sales lead if it has a 7x higher chance of creating a successful connection.