the more tired they are, the better. I've subbed decent fighters (Marcus Davis comes to mind) with a 66 sub off CAF after tiring him out for 3 rounds. My strategy for subs is this, ware them down as much as you can, ether on the feet or with GnP. Eventually, get the mount. Especially on higher difficulty levels, the CPU fighter will immediately try to transition out of the mount and give up their back, prevent this and hit em in the mouth, rinse, repeat until they are gassed, rocked, or even better, both. As soon as this happens, go for the sub and twirl that stick like your life depends on it.
It helps to have beat them up for q while, reducing their max stamina as low as possible. I actually managed to sub BJ Penn after wailing on his body the whole fight. In my experience, it also helps to attempt subs from higher up on the "ground pyramid" but that could be total coincidence.