It takes a while to digest.
It's really not comparable to GHB/GBL as it lacks the very short GABA-b agonist half life and GHB receptor agonist. Honestly the GHB receptor has a very broad spectrum of roles, hypothesized to even stimulate oxytocin like MDMA. It's also one of the hardest drugs to dose. (But I like that, it makes us G-people rare and undetected.)
Phenibut, I hear people going from 500mg to 17g.
Anyway, I think the main cause of phenibut's awkward reputation for being either amazing or "just alright" has a lot to do with metabolism and stomach contents.
Oral seems to be the worst ROA, however I have not heard any first hand reports of people snorting it. I also have not heard any reports of anyone passing out (involuntarily) on phenibut alone.
According to ebola! 750mg rectally is supposed to give a fast onset of 20 minutes and be a pleasant experience.
If there is a recreational dose, then there is also a toxic dose, which I really have not heard much of.
As for withdrawals and tolerance. Like with any GABA drug. You need to let it leave your system before you take it again. 24-7 dosing, especially with GABA-b agonists is a one way road to hell. I would give yourself 2-3 days minimum in between phenibut use.
I wish there were a way to decrease the onset time of phenibut, and increase the oral BA. I think it would be a lot more recreational if one could produce a rush with it. Perhaps first by figuring out how its metabolized. Please PM me if you know how.