Ok, so how many centuries would that be?
During the time of the Buddha himself.
Ud pg. 44:
‘‘Passathāyyā samaṇānaṃ sakyaputtiyānaṃ kammaṃ! Alajjino ime samaṇā sakyaputtiyā dussīlā pāpadhammā musāvādino abrahmacārino. ...
It's a very common expression. The above are just a couple randomly chosen.
Fair enough, but what what I was getting at was that since nothing was written down for centuries after the Buddha, the sutras are telling us more about the terminology used by writers two or three centuries after his time, than anything else. That's enough time time for the development of a complete religious vernacular, significant shifts in language use and so on. Of course I could be wrong, I seem to recall it happening once before...
I also seem to recall vaguely something about the Buddha telling people not to call themselves Buddhists, but rather followers of the Dharma. Do you recall any such passage? Or am I just pulling scripture out of the air?
The exact same terminology is also found in the the translations we have of the Agamas, from the Sarvastivada, the Dharmagupta, and the Mahasamghika. These traditions from the NW, in the light of the Mahavihara (ie. Theravada) tradition of the SE, split a little over 100 years after the parinirvana (not "two or three centuries"). Being written down or not may thus be somewhat irrelevant. It is a fairly safe call that the traditions from which they all came also thus shared the same terminology on a large scale. If this is on a large scale, it would take some time to be so imbued, the texts representing what was obviously extremely common place. Thus, it is a fair call that this terminology was used during the Buddha's own time. If separate religious terms appeared, they could only have appeared in individual (or perhaps a couple of) nikaya schools. But, this is not the case. Other methods of establishing original teachings, such as that used by Bronkhorst in his last few works in this area, would also support my position made here.
For your last point, please cite a text rather than vague unsupported claims, or throwing responsibility to find such "passages" on to others. Moreover, on the issue of being "buddhist", considering that this is an English term, one would first have to establish the supposed equivalent in middle Indic, or at least in Sanskrit. Keep in mind, also, that during the first early centuries after Sakyamuni, the term "buddha" was not confined to what in modern English is known as "buddhism". eg. the Jainas and other sramana movements used the same term. The adjective "bauddha" or "bauddhika", which is attested in the Atthakathas, for example, could really only apply to so-called "buddhists" after movements such as the Jainas were largely on the way out. See, for instance, the criticism that Gaudapada received about being a "crypto-buddhist" (pracchanna-bauddha). "Sakyaputriya" is thus a much more emic term, that has relevance to Buddhists themselves. And, as shown above, they really did often refer to themselves as "sakyaputra" and "sakyaduhitra", "sons / daughters of the sakya(ns)".