I think the smaller doses will give you physical effects that aren't really noticeable. Changes in heart rate and blood pressure, more energy and better mood. But, the effects will be too small to notice and will just seem like a day you are in a good mood, not like a drug produced effect. Low dosages ability to give you longer and more restful sleep is probably the one thing you would be able to look back on and say it definitely has an effect on, seeeing as you can look at sleep time/quality with a quantitative analysis. I know I rarely sleep more than 8 hrs and wake up atleast once at the 4-5 hr range(that's what I consider good for me). pheibut usually allows for about 9-10 hrs and good chance I won't wake up, if I do I fall right back to sleep, which isn't usually the case.
I think low doses tend to disappoint, too. I also never really started low. I think the first time I did something between .5 and .75 grams and it was hardly noticeable. But then I did around 1.5 and was very much effected by it. I haven't gone back to a dose lower than 1.25, I think that's around the baseline for getting the effects I desire. But, I also wouldn't go over 1.75. Also, I try to give it appx a week between use. I can probably count on one hand the amount of time I've done it with 3 or less days of not using it. Those times were probably when I would go the the higher end of my dosing(1.75), after doing 1.25-1.5. It would work, but the duration seemed to be much shorter each time. I think this is probably because of how phenibut is pretty harsh on the body in terms of sapping it of vital minerals/nutrients(I believe it depletes the body of potassium at a significant level, which can take several days for the body to get back to regular K+ levels).
Usually when a drug is depleting your body of something, then the drug is likely to have less of an effect if you take it while still lacking adequate amounts of that nutrient. Similar to how I feel about opiates and magnesium. Being an addict I have always felt magnesium depleted and if enough is lost you lose a lot of the positive effects you are looking for from the opiates...or it brings out negative effects(muscle cramps, twitching, anxiety and agitation).
About tolerance. I don't think I ever used it enough to build up much of one, as I said, I have (and still do but it's an off an on habit) used at most 2x a week and mostly 1x a week. Also that was for a period of 3 months stretch at longest, there have been several stretcher of doing that, though. Usually I would use it to deal with stressful days(basically long shifts at my job) or for recreational days, almost exclusively have used it when I've been using Suboxone for maintenance of my opaite addiction and looking to add something to the lifeless, boring "high" that Subs give.
Surprisingly I never got into daily, or even every-other-day use, even with a very addictive personality. I could tell it was just real rough on my body and I needed the 5-6 days off to not beat my body down. Kind of like alcohol, it gives a hangover. Bit it is hardly as obvious and not as miserable, but it lasts longer. Like a vague depression/lack of energy or motivation that lingers for 2 days after use. Allso, you are definitely dehydrated and lacking some important electrolytes. This was quite pronounced in times when I did it 2x in a span of 3 days, which like mentioned, was a rarity.
I don't know if I have ever gone above 2 g in one dose(def have in a day, example - wake up for work at 6AM and take ~1.5g, then another .75-1g at T+12 hours...I work 24 hr shifts so want to give it that extra bump to get thru the shift, typically I'd take it on the days I worked my 24). I can say after several months w/o doing any, I'll get basically the same effect from 1.25g that I did the first time I did it, no change in intensity or duration. Even when using regularly at doses in the 1.5g range, but spacing out doses by appx 1 week, I've never noticed much of a difference in the effects.