Because they are SO spendy, I don't change mine very often, I flip them when it gets hard to cut with and that makes it last a little longer...... I got a package of five for Christmas and changed it then and just changed it again last week, normally I would probably go 4 to 6 mos without changing it.... but I cut four large quilt tops out with that one blade so it went fast!
I have saved all my old blades in hopes of buying a blade sharpener some day!