Tolerance to different effects occur at different rates. Respiratory depression, I've read before (though the source isn't necessarily 100% accurate) does increase in tolerance more slowly than euphoria. But that doesn't necessarily mean it reaches a ceiling at the same point (maybe earlier than euphoria), nor does it mean that respiratory depression tolerance can't catch up to a stable dose level. I have read of cases of an addict taking up to 2 grams of pure in a clinical setting "without any ill effects", though these may be exceptions to a rule.
Prescription of Narcotics for Heroin Addicts: Main Results of the Swiss National Cohort Study
Volume One. Uchtenhagen, A;Dobler-Mikola, A.; Steffen, T; Gutzwiller, F.; Blattler, R; Pfeifer, S. Karger; Basel; 1999
Page 20 states that the mean daily dose of heroin IV ( when used alone, without methadone on the side) was 491.7mg.
Page 22 states "A stable dose was achieved after 6 months at most; beyond this point, almost no further increases in dose were required."
Later in the volume, the authors state that most users tend to gradually reduce their dosages after achieving a peak.
from erowid:
http://www.erowid.org/chemicals/heroin/heroin_dose1.shtml
So I'm guessing that the euphoria tolerance reaches a ceiling rather than reaching a point where you would be overdosing to catch a buzz.
I wonder... how does it feel to be at the euphoria tolerance ceiling? Does the typical user feel just a little high when shooting, and a little sick for most of the day? The author of the "Heroin User's Handbook" wrote (IIRC) that he mostly felt slightly ill, rather than always slightly high. ("why addiction sucks" ... he suggested that chipping was a more effective way of being high on heroin on a regular basis)
I wonder if a maxed tolerance still has any other potential mental effects, such as reduced anxiety, OCD, or depression? Part of what I love about opioids is that for a couple days after a single dose, I feel a little calmer and relaxed, physically and mentally. It's a very small effect, but useful in my case... a medicating effect beyond pain relief and euphoria.
I wonder if it's possible to indefinitely be high every day, or every other day for instance. By having a maintaining dose most of the time, to keep sickness at bay, while taking a strong dose just once every 24/48/72 hours to get high.
What might be theoretically possible is being high twice a week, for the rest of your life. Waiting at least 3 days between doses might be just about enough time for the drug to totally leave the system and the brain to re-adjust without it. A person would have to go by how their body felt... if they get a mini withdrawal (barely noticeable, for example), wait until it has fully passed until using again. Or just use every weekend... though maybe that would create an unpleasant mid week!