Plan B effectiveness?

Long story short, happened with a spermicidal lubricant condom which came off during at 2am, we bought the pill and she took it at 9am right when the pharmacy opened. My question: How much more effective is it to take Plan B with 8 hours of ? Does it make a difference whether it's taken within the first 8 hours compared to taken around the 70th hour, which would be the last possible moment if I'm correct? Thanks