I want to say "Let's work this later, "see you/so long" I don't know which one is more formal and I don't know which one take more time to "meet someone again"
"See you" and "so long" are roughly equal in formality in American English. Both are what most people would probably call informal - you can use either as a sign-off to coworkers of equal rank, and to immediate supervisors with which you have friendly interaction in any typical office environment. For anything farther off, you'd want something more formal such as "sincerely" or "signed". (It's also common to omit a signoff altogether in email, and just use a signature file.)
"So long"'s origin is unknown, though there are a few candidates. Nowadays, it's identical to "goodbye", "farewell", and several other parting phrases.
It would be odd to inquire how long it would be until next meeting, unless there were another reason to ask (i.e. you need to meet again to follow up on some issue). In all cases, the choice of signoff phrase is independent of time until next meeting unless you explicitly provide a time (e.g. "see you tomorrow"; "so long until next year").