These expressions are very common in U.S. spoken English. They are slightly informal, but not very. I would write them with a hyphen: one-ish, two-ish, three-ish, four-ish, five-ish, six-ish, seven-ish, eight-ish, nine-ish, ten-ish, eleven-ish, twelve-ish, for two reasons. First, because they are not "real" words, i.e. they are not in the dictionary; second, to clarify the pronunciation in the case where the number ends with a vowel. The native English speaker, seeing "twoish," is distracted for a few milliseconds by the possibility that the "oi" is a diphthong, as in "boil" or "android."

Hi Igor, When speaking to someone in an informal setting if they are familiar with English you could say one-ish, two-ish ect. However, when speaking in a more formal conversation or if sending a message to be read it is best to rewrite the sentence as, "I'll be there around one o'clock." One-ish, or two-ish sounds too conversational when read in say an email for a job interview or any letter to someone you may not know. Chris

I wouldn't really use it in written English but it is very common in conversational English (at least in England it is). If I was writing to a friend I'd use it but "I'll be there at around one/two" would be slightly more formal.

Speaking in this manner is very informal so I don't think that anybody can say that there is a right or wrong way to write such words. I would be inclined to put a hyphen in them (for example, five-ish) but I won't say that's the correct way. It's my opinion that "around five" or "about five" is more proper.