How old is English?
 This is certainly true for well-established and dominant peoples in Europe. The French consider their language as the most cultured in the world, so do the Germans and of course the English. We can also speak about a French language universe, an English language universe, etc. People within such a universe have difficulty imagining that there are other languages in the world. Everything clearly happens within their own universe. Only smaller populations, who have more tradition in foreign languages, are more likely to escape from this rather narrow view of the world.
 S. Oppenheimer : The Origin of the British. e.g. page 146 and others
 C. Julius Caesar : "De Bello Gallico". Commenting the conquest of Aquitaine by his general, he wrote: "The people of Aquitaine can give the Romans lessons in good Latin".
 see: Wikipedia article on Occitan language - the version in French is the best.
 meaning: close to Brythonic
 There are more contemporary texts about the transition to a Roman language.
 Let's not exaggerate: the total vocabulary probably didn't exceed 10 000 words.
Most peoples believe they are the center of the world. Other peoples are considered less valuable, so any foreign language is second-class to their own .
Dominant peoples gladly believe that others should change their language into … a more civilized language, which is of course their own. Some Americans believe that the main language in India or in Hong Kong is English. But that’s not true. It is true that many people worldwide do speak English as a first language, or learned English as a second language. But this does not mean that this foreign language is used on a daily basis in the kitchen. When a language is spoken in the kitchen then one knows that it is still very much alive.
It is not easy to force entire populations to change their daily language.
However, there are some examples in history of populations who changed their languages.
How long does a language need to change completely? Note that languages evolve but never into an imposed, foreign language. A foreign language has to be learned.
Imposing a language seems to be no longer fashionable.
Historians and linguists generally believe that Latin was introduced into Gaul around 50 BC, and evolved into French. This theory is probably not entirely correct. Genetic studies  reveal that the region around Paris is genetically distinct from e.g. Brittany (France). The whole south of Gaul (roughly south of the Loire) spoke Occitan before the Roman conquest . Occitan is so close to Latin that Latin itself can be considered to be nothing more but a Occitan dialect . It was most probably this Occitan which was introduced in the north of Gaul and not Latin itself. The local para-Brythonic  language changed into 'northern-Occitan' and became as French the language of the vast majority in the North around AD 850. In northern Gaul, Occitan was not imposed, but widely accepted. The Gaulish aristocracy organized local schools for their children. Schooling was not state subsidized until the end of the 19th century and had in most cases to be paid for privately. Less fortunate people had no access to schooling. The Gaulish aristocracy used the 'new' language early on to prove that they were educated, that their parents could afford schooling. In 486 AD, Clovis and his Franks conquered Gaul. Although the Franks spoke a Germanic language (proto-Flemish), they didn't manage to change the language in Gaul. The present language transition simply went on. How can one explain that the Anglo-Saxons did succeed in changing the language in Britain?
The transition needed at least some 900 years to become complete (54 BC - 850 AD). In the 5th century, Sidonius Appolinaris, a Christian missionary, complained in a letter to Rome about the fact that half the population in Gaul couldn't understand his (Occitan) sermons. 
The original language of northern Gaul remains partly a mystery. A likely candidate is a Brythonic language, but remote from the language spoken in Armorica (Brittany) and West Britain, mixed with strong and early Occitan influences (maybe 50% of all words) and probably also Germanic words  . Some words, now qualified as francique , could have been present in the language before the conquest of the Franks. The intimate relation of para-Brythonic with the southern Occitan language may explain why the language change happened apparently without resistance. The German words were suppressed in official texts, as those texts had to be written in the official Occitan dialect: Latin. Later, those Germanic words, already spread within the population simply came back. Forget the idea that French is derived from vulgar Latin. There was no such thing as vulgar Latin. There were only other Occitan dialects. So, the language gap was less wide than initially supposed.
The more an oppressed language is close to a new one, the faster a transition will happen. It is feasible to learn a close language without lessons. Native Dutch speakers can ‘pick up' German (or English) but not French. Since the 16th century official national languages, along with a fixed writing standard, were introduced in Europe and they had little trouble imposing themselves upon the local dialects. But when the gap is much bigger, like between French and German, or Welsh and English, things are less simple and much slower.
800 up to 1000 years seems to be
the average time period of the transition from the
original local language to the new imposed one.