The earliest evidence of the forgery operation is found in a report made to Spanish authorities in 1796. That is coincidentally the year that England declared war on Spain. Spain had just come under the control of Napoleon and that caused Spain to "switch sides" in the war. Before 1796 Spain and England were allied against Napoleon. Subsequent reports indicated counterfeits using 15 different dates were discovered, but when all these dates were first noted is unknown. The initial report did include several "different" varieties were observed based on the method of manufacture.
So I think we are forced to conclude that the forgery was known before 1796 but by that time it was an already well developed (but not necessarily unified) operation. I suspect the effort to forge 8Rs before the declaration of war in 1796 was undertaken by trading rivals of the Spanish interests in the orient (East India Company?). The earliest types of counterfeit 8Rs tend to be somewhat more crudely executed possibly pointing to a "cottage industry". The number of variant technologies also points to several unrelated groups. But the earliest crude types are by no means common. This leads me to conclude that production was fairly limited before 1796.
However, starting with a group of coins dated 1792 the quality of the dies improves and the appearance of the coins and the numbers known to exist increases dramatically. This is (I believe) the point at which the larger manufacturing entities in Birmingham joined the forgery effort. The counterfeits involved are primarily dated 1792, 1794 and 1796 (they seemed to like even dates) and they all share similar die characteristics. Several reverse mint types were used Mo and So being most common. Other related dates do exist but they are all very scarce to rare.
So I conclude, that from the start of the War (1796) onward the forgery efforts seem to have had at least tacit support of the English government as a war effort attempting to destabilize the Spanish economy in the orient. Here too the East India Company is suspected of being involved but after 1796 they could have done so more openly. Before 1796 the forgery had to be secret because Spain and England were allies - but after 1796 .......
When the forgery operation ended is far less certain. The very common dates 1792, 1794 and 1796 are also known with the Bank of England counterstamps. So at least some of the forgers were not above targeting England itself. The Bank issues circulated rather briefly but are still very common and are often well worn. They also produced the 1804 Bank Dollar in large numbers. These coins are virtually all Sheffield Plate technologies. The silver plates vary in thickness and later developments including a white metal core (instead of dark copper and bronze) indicate a rather long period of manufacture. The thinner the plate - the later in date the counterfeit.
The third phase of this forgery effort actually targeted the English Colonies in the Americas (US, Canada and the Islands). This is in my opinion the reason these counterfeits still exist today in substantial numbers. England restricted silver imports into its colonies so they were ripe markets for forgeries. The colonies needed specie and even counterfeits were better than worthless paper notes. It was during this third phase that most of the "wear" observed on Portrait 8R counterfeits occurred. It is OBVIOUS based on average wear that many of these coins were circulating long after the fraudulent nature of the coins was visible. So the prolonged use of the forgeries as "Tokens" could have lasted into the period of the US Hard Times.
Starting in the 1830's technological changes and discoveries in metallurgy altered the way forgeries were made. By 1850 or 1860 counterfeiting was totally different - dies, alloys and striking methods all changed.