Yeah, I could use 10ms to avoid any noticable flickering, but that's on an I7 processor. To be safe on slower computers, I would guess that 40ms would be OK. I'd suggest making a stress test in 1 core mode while using a couple of heavy VST/VSTi:s.
Ok, I'll try it on a laptop. My main PC has an i7. I'll have to test things on 2 PCs, one fast and one slow.