How long ago did Londoners start saying "f" instead of "th"?

Solution 1:

Early usages date to the late 18th century according to the following source:

The first reference to th-fronting is in the "low English" of London in 1787, though only a single author in that century writes about it, and it was likely perceived as an idiosyncrasy, rather than a full-fledged dialect feature of Cockney English, even into the early half of the twentieth century.

The feature was presumed to be reasonably common in London speakers born around 1850 and in Bristol by 1880. The use of the labiodental fricatives [f] and [v] for the dental fricatives [θ] and [ð] was noted in Yorkshire in 1876. In his 1892 book A Grammar of the Dialect of Windhill, Joseph Wright noted variable th-fronting in his district in words such as think, third and smithy.

(Wikipedia )