grzbear
My first "scoby" took about 2 weeks to form adequately (I do not remember how long exactly, may have been a bit longer)... I do remember that I could not drink the first batch... too strong. From then on, it has taken 7 - 10 days dependent upon the room temperature.
I have never used "decaffeinated" tea cannot help you there, though I do not see why it would not work.
I would think it to be fine.