That slow would be possible across a problem zone, and even then only with long stops. 100m in 3 weeks would be more like it under a city like Paris, or even multiples of that.

Nowadays, unless facing unexpected geological problems (see the ill-fated North-South metro across Amsterdam), tunnel boring itself is not even the majority of the time of construction: there is surveying before, and elaborate tunnel fitting afterwards, and then track commissioning. For example, Paris's line 14 was dug for two years (at a rate of 350m/month), but opened three and a half years after tunnel boring was finished.

how fast can demand for mobility channelled to new public transportation

That's too broad a question... always depends on local circumstances, and what you mean by "channelled to new public transportation". (Do you mean how long it takes for inhabitants to switch to a new project? Or how long it would take to get a majority of them to switch to public transport? Or 100%?)

_{*Lunatic*, n.
One whose delusions are out of fashion.}