Tolerance is an effect well-described in literature, it happens with most drugs and it is an exception when it doesn't happen. If you want evidence, check the following wikipedia pages and their reference articles:
http://en.wikipedia.org/wiki/Drug_tolerance
http://en.wikipedia.org/wiki/Desensitization_(medicine)
http://en.wikipedia.org/wiki/Downregulation_and_upregulation
In pharmacology, desensitization is the loss of responsiveness to the continuing or increasing dose of a drug. Also termed tachyphylaxis, downregulation, fade or drug tolerance. This may be an important area to consider for the future design of safer drugs
While tolerance may not be terribly consistent, it does happen with psychedelics. Cross-tolerance is even less predictable. It isn't black and white, like you absolutely can or absolutely cannot trip in the days after having taken a psychedelic. It depends on a lot of factors, among which the dynamics of desensitisation and down-regulation of 5-HT2A receptors. I think that the harder you trip the first time, the more pronounced the tolerance will be and the less effect you will get in the following week. That is not to say it is impossible to trip, you just need more of a drug to get a similar level of effects! That is why while tolerance or 'resistance' to drug effects might get stronger the more you try to use a drug, you can compensate by taking an increasingly bigger dose.
Whether you can trip for a month is hard to say, I would bet good money that the drug's effects will always change even if achieve a persistently strong reaction. Psychologically you will always get used to it, and your consciousness will always try to balance things out somehow.
It will pretty much vary between individuals, if only because there are often differences in the translation from pharmacological action to subjectively experienced effects in an individual. But sooner or later, in some quantity, people observe tolerance effects with drugs including psychedelics. However there are some weird cases like DMT that appear to have a different rate of tolerance setting in, it probably has more to do with the duration of one or multiple trips than the number of times someone trips.
The reason why there can be huge variation in how long people will tell you to wait is because tolerance returns to normal at a fast rate at first but it goes slower and slower. It may technically never return to normal, I'm not sure about what statements we can make about that. But how long to wait depends on what people consider to be a return to normal that is close enough so that we can stop calling it significant. After about a week you can trip again but it can have less impact because the impact from the first trip is still doing its thing and you haven't forgotten that yet (see, also a psychological component comes into play), but in most cases you can still get a similar level of psychedelic effects. So while psychedelic tolerance might take a week to a month to reset, psychological tolerance which is of course not a pharmacological factor can take much much longer to reset.
Skepticism is appreciated but I think the theory for tolerance and the observations in practice are so abundant, that it seems silly to dispute. And part of the apparent inconsistencies come from a poor understanding of the ways tolerance can vary, and the difference between physiological tolerance and psychological tolerance.
