I did a quick test to see how many characters show in a subject line. With desktop apps and web-based email clients, it’s not too big of a deal, and the old standard of a 50 character max seems to hold true. That is for multiple reasons, but as far as the space, 50 is fine.
For mobile it’s a different story. This is not scientific, but for the Gmail app on a Samsung Galaxy S4 (Android Kit Kat) the subject line max was 27 characters with the font size set to “Normal”. Of course that changes if the font size is changed. Setting it to “Large” takes it down to 20 characters, for example.
I check it out on a friend’s iPhone 6S and that displayed about 40 characters. That was in the Gmail app also. I am not sure about the OS or the font size setting, but it looked to be the default setting.