Time between on campus interview and tenure track job offer
Unfortunately, there is no clear and well-bounded answer.  In general, however, you should anticipate the time to be on the order of months.  

To understand why, consider these typical components of process:


After your interview, a number of other candidates may be coming to interview.
Once all of the candidates interview, the faculty needs to decide who they want to hire. This may happen in multiple stages (e.g., first a search committee triages down to the top candidates, then the full department chooses between them).
A formal offer is prepared for the top candidate and negotiated amongst the department and any other involved university organizations (e.g., HR, deans, centers, etc.).
The candidate has some weeks to decide whether to accept the offer, possibly including more negotiations and revisions to the offer.
If the candidate says no, things begin again with offer preparation, negotiation, and decision for the next candidate.


Every one of these steps is likely to take on the order of weeks, so you can see how it can easily add up to multiple months.