All the road running CD and tour were announced in a very short time and, if you think about it, as long as venues and the band are booked, there is no reason to put tickets on sale half a year before...
The rare is when tickets go on sale so soon, you never know what would happen six months later, so maybe you bought tickets and something happens in that six months that make it impossible to attend.
When tickets go on sale two months before the tour starts, you kind of know for sure if you can't go or not...
Dylan fall tour on 2011 was announced in July if I recall well and tour started on October...
Between 2 and 4 months are enough, and lately we are buying tickets before we even have the new album so very easily on February a record release date and tickets on sale can be announced for April (the CD) and May (the tour).