Hi,
You don't need to be logged in - it'll resume by itself (as long as the website has more than zero visitors).
A tiny site may finish backing up in 30 seconds. A very large one may need an hour. A gigantic one may need two, depending on available resources.
In a future version we'll get more visual feedback on what's going on under the bonnet... the log file is helpful, but also tends to scare people prematurely. Basically, wait an hour and then let us know then if it still looks like it's not finished (though I'll not be online until Monday).
David