If you send a very small fee then the confirmation time can be extremely long for some people but we must remember that we can adjust the rate of our fee so if we need a fast transaction we can always send a bigger fee and get confirmation in minutes.
by guessing.. since there is no real way to know how much fee you should add.. unless you just add some huge amount.
i sent a tx with .0003 fee and its been 9 hours.. that should have been enough fee for a few minutes, at least it has been in the past..
There is a site that allows you to see with 90% confidence how long your transaction could take depending in the fee you send, I don't post the link because I wont click on a link posted by someone I don't trust, but you can find it very easily by looking for: bitcoin fee transaction.