As was already answered, the limit on titles is 150 characters. We just released a new feature network-wide (including Stack Overflow for Teams) where users will be shown a character count starting when the Title hits 100 characters (this was originally requested by the Mathematics meta community in 2012…err…6-8 weeks ago).
The count will appear above the right end of the Title input box and will function in the same way as the character counter used elsewhere on the site (most prominently with comments):
When you go over the limit, an error state will show on the box. The character count will update as changes are made to the title. The counter will count actual text in the box (so if you are including something like LaTeX in the title, it will count the number of character before rendering it).