In article , Gaiawar
writes
It varies widely. Apparently for humans & other primates it took millions of
years. For orchids & some other plants which colonize a newly disturbed site
in
the jungle, it may only take a few generations.

Years have passed during my search for a citation of such an event.
What is your reference?

Perhaps Senecio eboracensis (see http://www.bsbi.org.uk/Volume_24.htm).

You might find something in Verne Grant's "Plant Speciation".

There's supposed to be new orchid species which have appeared on coal
tips in central Scotland, but I wasn't able to find a reference in
Google.

There's always polyploid speciation, which takes place in one
generation.
--
Stewart Robert Hinsley