I am by no means an expert, however I started using BT again the same night I got my relaxer. I knew that the next day I would be washing my hair, so I figured that it couldn't be that harmful. Everything turned out fine for me.
I did stop using BT about 10 days before I got my relaxer, but I don't think you have to wait any amount of time after your relaxer. Now, of course, if you were burned from your relaxer or have a sensitive scalp you may want to wait.