With the 15:15, which is wiser for beginners, you will be using a heavier KB (eventually) than with the 36:36 purely because the 36 seconds straight will take a whole sight more energy...it also means that having done the 36:36 you will be able to go up a size in KB if you choose to do the 15:15 at some point afterwards and so forth.
Dan John also does these, the 36:36 protocol, and from what I recall does not take it to the 80 recommended sets, which would take around 40 minutes at the 15:15 protocol, but goes to around 20-25 minutes total work time which is all anyone really needs.
Those attempting the 80 sets are generally after an RKC certification and need the stamina for that anyway which this provides for them.
Enjoy, but build up slowly, remember your skin has to adjust as well.
For the 15:15, I use a digital kitchen timer, since every 15 seconds, I'm either picking the KB up or putting it down, so there's not a lot of mental focus required to work out what I'm supposed to be doing and when; start at 15 seconds, that way you don't faff about trying to switch it on and pick up the KB at the same time.
36.5 kg /80.3 lb Middle-Finger DL (right hand)...