I maybe getting ahead of myself here....

A lot of it isn't even praciticing learning by ear, it's just practicing normal stuff on the guitar. Eventually you come to a point where you go "that note sounds like its here" and you play the note. Eventually, after you play a lot, you start to remember key phrases players use, and it's real easy to pick those out and play them when you know the key of the song. Lots of rock n'roll/blues playing works out of a box, so if you know the box, you can guess pretty easily what the next note will be.

Thanks Taso !