Yes, this is very true. In fact, I've said almost the same thing in another thread before, and I've seen similar opinions expressed very widely. This is why re-dosing can cause such nerve-racking side effects, if you re-dose as the euphoria fades. The stimulation stacks, and stacks, and stacks.
I usually begin to feel an uncomfortable level of anxiety (and the need to chase that anxiety away with re-doses) at or around 15 - 20mg consumed without rest in between. Sometimes it comes sooner, sometimes later. Unfortunately, I feel that, once this line is crossed, the easiest and most tempting way to subside the new found anxiety (temporarily, of course) is to re-dose, there by initiating the cycle.
A good motivation to stop is thinking about whether you wish to obtain sleep the coming night / morning.. If you do, try to plan that at LEAST 6 hours after your last dose getting to sleep will be very difficult.