When is it appropriate, if at all, to use the suffix "ish"?

Solution 1:

I think you could make the argument that the -ish suffix should not be used in formal English to create ad-hoc words on the spot, like "largish". There is nothing grammatically wrong — you will, of course be understood — but it has a definite informal connotation. However, there are a number of established words, e.g. impish, boorish, devilish, sheepish, etc., where -ish is accepted in all contexts, including formal ones.

Solution 2:

Adding the "ish" suffix to a word X is a colloquial way to say that something is "somewhat" X, "approximately" X, "rather" X, etc.

1. She was a largish woman. ⇒ She was a rather large woman.

2. She will be here by fiveish. ⇒ She will be here by any time close to five o'clock.

3. The boy is fiveish. ⇒ The boy is five years old or about that age.

4. The interior has niceish plastics covering the dash and the doors ⇒ The interior has rather nice plastics covering the dash and the doors.

5. The exam went well, ish. ⇒ The exam went fairly well.

See Wictionary page here.