When can one speak of chromaticism?

It depends on the context.

Chromaticism means many things. It is already present in the late Middle Ages/Renaissance with musica ficta and the progressive sensitization of cadences. It is in the late Renaissance, e.g., in the madrigals of Gesualdo, Monteverdi, etc. It is in pre-tonal and tonal harmony from 1600 onwards. It is in late Romanticism (Wagner, Scriabin, Reger, etc.).

Thanks zedef, I was probably too vague

I was being more down-to-earth, let's take a chromatic scale C C# D D# E F etc.

It's chromatic because there are chromatics C C# ... C# D ...etc.

So in this case is it correct to speak of chromaticism?

C do# is a chromaticism, C# D ... no

Chromatism in the strict sense is altering a note while maintaining its "base name". If you are in C major and you have a G, chromaticism means having it followed by G# or Gb (one speaks of a chromatic semitone). When instead you alter it by changing the name (e.g., G-Ab or G-F#), one speaks of a diatonic semitone.

If you take the natural minor scale, to make it harmonic you chromatize the seventh degree in an ascending sense (e.g., in A minor, the G becomes G#); thus, to make it melodic, you chromatize the sixth and seventh (F# and G#, only when ascending... when returning they become natural, i.e., diatonic).

Then, extending the concept, everything that moves by semitone, whether chromatic or diatonic, is commonly called chromatism. From this comes the chromatic scale.

Chromatic scale is one thing, chromaticism is another. As Zedef rightly says, one must understand what is being referred to. Obviously, C-C# is a chromaticism built on a chromatic semitone; C#-D is diatonicism, built on a diatonic semitone.

Ok, thanks guys. The matter is clearer to me now.