I don't have an exact time for you and it would depend on how many connections you have set along with your DNS etc... Google DNS or similar might be stronger for you then standard DNS of your ISP. I know my home isp is comcast and their DNS flat is poor. ATT DNS is great. At my office its pretty solid as well, but its business class. Also servers, if they are using a data center DNS is usually pretty solid.
So it just varies, but the alive checker does header checks first and then if it fails the header it does a GET so unless you have a load of dead urls in the list then its probably going to be pretty fast.