i2b2 allows labels that are supposed to be shown to the end users to have a length of 2000 characters. This would be equivalent to nearly one page of text and is of course unsuitable for the limited space on a standard computer screen.
It's better to limit the designation of a concept or folder to 40 characters at maximum, most can be even shorter. But be precise: it's better to label a concept "Date of admission to the ICU" rather then "ICU admission" or "admission date" because every concept should be understandable on its own.