There is no character limit when it comes to customising labels. You can enter any text you desire.
However, if you notice that the text you typed does not appear completely in the preview window is due to the fact that you have entered too many characters and the text will turn out too small to read. Font sizes are automatically adjusted to the length of the text that is entered in the editor field. In any case, the labels will be processed with the text you typed. Please be sure you are happy with the appearance of the preview text before completing your order.