<What determines when a word is in currency as a trademark and when it has seeped out into wider usage? >

Let me just say that this is a fascinating area for trade mark lawyers.

In Australia, a registered TM is liable to be struck off the Register if someone can show the court that it has become "generally accepted within the relevant trade as the mark that describes or is the name of the article, substance or service" for which it is registered and used. That is a much more onerous task than showing that mere consumers apply or regard it generically.

All the major corporates keep a very watchful eye on uses of their TMs which may dilute their value as TMs, eg. describing someone as delivering "Rolls Royce service" is a no-no so far as the RR Motor Co. is concerned.