Typical is the key word here. 30on 30off is something I consider to be at the top of my Vo2Max work - way more pain than 3-8 minute efforts for me - working the same system in a different way. I have never tried 15/15, but that seems to just be a different way to do the say thing ... if repeated enough. The key is level of effort, so if you are not as well trained a 15/15 would likely be the same as a 30/30. although I would be worried about too much ATP stuff at the 15/15 level I am always mixing up my training systems. I love tossing 10-30 efforts on top of 10 minute FTP work or 5 minute VO2 Max work. Makes it more race like to mix up your energy systems.