It varies a lot depending on how loose you like it. As just an estimate we'll say around 400 lbs of tension for a medium tight line that is around 30 to 40 feet, but they get much looser and much tighter than that and the longer it is, the more tension is required to keep from bottoming out. That is just to hold the line tight but when someone is on the line physics amplify that number by a good margin, which is why it is necessary to have very solid anchors. Eric Matthes has a really interesting Technical Analysis of a Slacklines on his site. He calculated the amount of tension put on a slackline by the amount of sag. I highly suggest technical info geeks check it out. We simplified his equation into a Slackline Force Calculator.