No dose would act the same day for me in case of phenibut. I'm sure if I took a massive overdose, especially by say, plugging or IV (not that I'd IV phenibut. I don't even have a desire to take it orally) that it could do so. But, if I were to take it, I'd have to take it about 24 hours before the point I wished it to take effect.
I should add, I am a total nonresponder to oral baclofen. Some people find that it simply cannot work by mouth but it does via intrathecal spinal pump implant, others find that intraspinally it produces no effect but it works by mouth. I've never tried it via an implanted pump and don't want to have anybody fidding around with my spine either. It isn't that I'd have to wait for baclofen to kick in, it just won't kick in. Not even at dosages where a handful of the 10mg pills is tipped out in a heap into my hand and chased down with a drink. At most, a couple of hours later my eyes will feel a bit swimmy and wet, vision-wise, and it can make me mildly nauseous, but otherwise it has no more effect than a teaspoon of distilled water or an injection of a ml of saline, whilst knowing exactly what it is.
For me, although I've never tried fluorobaclofen or bromobaclofen/iodobaclofen etc. these drugs are garbage for me. I should also add that gabapentin, even taken stepwise, its pretty useless to me.