I know in the SF3 series they had the stun meter which let you know how close you were too dizzying your opponent. Did they implicate a way to know in SF4?
You just have to keep track of it in your head.
You can watch the stun counter in training mode to get a sense for how much time can pass before stun starts depleting.
There is a hidden stun meter/counter in the game, unlike the more obvious one in 3rd Strike. Certain moves inflict more stun that others, like some inflict around 20, 30, 50… while some heavy hits can inflict around 250, etc. Also know that those values of stun for any move are subject to scaling just like damage scaling. If you are familiar with the amount that the engine scales damage for hits, then it works for the some reduction scale for stun.
Finally, it can usually take several combos to inflict a stun state, but know that if you can just keep them blocking, the stun meter will not go down for them. The stun meter only goes down after so long without going into a block state. A good rushdown will ensure that there stun meter doesn’t have the chance to go down, so you can eventually wear at them till they get stunned.
Usually, one or two fairly long, decent combos are enough to inflict enough stun on an opponent, granted you don’t allow his stun gauge to descend. That’s how most people keep track of it, you can get a feel for it eventually.
You can check the stun points in training mode, but otherwise it’s hidden.
I believe most normal attacks deal the same amount of stun (weak = 50, med = 100, strong = 200), while specials vary. Most characters have average stun (1000 points), so with this info, you can get a general idea of when you’re about to stun your opponent.