For blues you practice the Blues Scale, if you know it all ready there is no point in further waiting. You may start improvise whenever you like.
If you don't know it, practice playing it without the bent tones.
Beside that you should listen to a lot of blues, and you might want to watch Adam Gussows video on counting your way through the 12-bar blues (I think it is vid 0.016 - 0.019).
To sharpen your ability to improvise in general, I recommend you play the major scale in different positions. Try playing it up and down for a while, and when you get comfortable you start to alternate the tones. It's difficult to explain in writing, but the fundamental idea is that you think these things out for yourself. Play the scale, and after a little while allow yourself to experiment with the way it is structured. You can't go wrong when improvising
