I don't think it's anywhere near an equal amount to get into fatigue as it is to get to 100%. I always hit fatigue the same day or next day after it resets, and it goes up pretty slowly after that. It feels more like it goes up 10% each time you hit that same amount.
As far as reseting, it resets a week after you gained your first SP in a given job after the last reset.
So, if your PUG reset on Thursday at 10pm, and you started playing your PUG again Friday at 2pm. Then hit fatigue on Monday, it would reset Friday at 2pm. Each job has its own timer. EXP probably works in the same way, I've just never paid attention to it.
To be honest though, fatigue isn't a big deal. I started hitting it in 1-2 days around 25 and have ever since, and I just play through it. I mean, you figure if you're hanging out w/ friends doing leves, and you're 3 tiers deep into fatigue, and they all get 100k SP, you still get 70k. Which is A) more on the class you want to rank up, B) more than you'd get soloing on some lower job class because you felt forced to,and C) more fun, because you're playing with your friends. That's all invalid if you have another group of friends that are a lower rank though :)
I haven't done steady SP grinding with the new SP changes yet, which of course means everyone can fatigue faster, and I might get past that 50% mark and my attitude might change, but your numbers turning yellow isn't all that big of a deal, especially if it's happening to you halfway through the week. Just keep playing like normal and ignore it.
Something else to note, your yellow numbers indicating fatigue will never disappear until you kill something/craft something (increase the SP value) on that class. So don't just switch to check if it's gone yet. In the example above about the PUG, when he switched to PUG on Friday, his numbers would still be yellow, but when he killed something, he wouldn't receive the bonus SP message, and his numbers would instantly turn white, which might make some people believe the act of killing something brought them out of fatigue, but it's just an interface bug. It doesn't get told when fatigue resets except by -not- getting that message after a kill/craft, apparently.
I'm on a week away from any comp that can handle FFXIV, so when I get back all my jobs will be out of fatigue and I plan to A) confirm that the reset happens exactly 7 days after I start playing again and B) confirm the amount of SP it takes for me to hit fatigue. (Going to see what it is on my 44 ARC, and my 6 LNC I think, to see if there's a difference based on rank.)