The stored limit is 50 characters, which is quite a bit for a tab as is, but you can surpass that by using a language string then translating the string to an unlimited length string. Example as follows.
Title: MY_TAB_1
Language Override
Key: MY_TAB_1
Text: My tab 1
See the below on how to use language overrides.
www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived