ikode, i'd use the rhizo more than a week, but it's dear, and tbh i get better roots using seaweed extract, and that only costs £4.50 for 800ml from wilkinson's, rhizo is £12 for 250ml.
The seaweed goes in at 5ml per litre and i have NEVER had roots as good as what i got now using it, my rhizo is now sitting un-used, will do a side by side with 2 cuts when my next psychosis root, as for using it a week, i use it all way thru veg, and lower to a minimum dose around week 5 of flower, cutting it off only at week 7.