I honestly can't say that there is a "correct" time to play a 7th or 9th chord, however there are many tendencies throughout blues music that I can draw an answer from.

1. If you are playing rhythm and there are many other instruments accompanying you...piano, another guitar etc. you may want to be careful with the application of 7ths or 9ths because they may interfere with the overall "musicality" of the piece. You don't want conflicting melodies and 7ths and 9ths tend to do that if there are many other instruments involved.

2. However the application of these chords are greatly valued in playing where the guitar is the lead instrument. The principles of Voiceleading and creating melodies can be complimented by 7th and 9th chords, or atleast when the added notes are needed in the melody.

These are just a few things, but I hope this helps. Peace.