I'm having a conversation with someone about this. The guy I'm talking to says that, no matter what, you should feel lactic acid buildup after 30 seconds. Either I don't know what that feels like or it doesn't happen to me. I don't think I have ever heard any of the other fighters in my gym complain of it, or shows signs of it, either. I do feel lactic acid after holding thai pads for a while, especially when working with heavier strikers. I normally get paired with HWs and holding pads for them, especially when working in counters, makes my arms tired.