How many are too many interviews with one company?

No one can decide that but you, but realistically, how serious are they about hiring anyone if their process takes that long. Perhaps they are interviewing multiple other people or are looking for additional candidates, hence the need to delay. Hopefully not for your sake, but it would make sense. Whether you pull out of not, good luck! 

I've seen entry level positions have as many as five interviews and senior level positions have as few as one. So I would say the number of interviews varies for what seems to be no good reason. And in terms of the length of time between interviews, that seems a little odd, but I have heard of companies having a year long interview process. And I have worked with clients who months after sending a resume will ask to do an interview. On the other hand, it seems very unprofessional if an employer does not follow up with you. In the end though, only you can decide if the position is worth it.  

It's an interview and a connect. While I would not plan on an offer, your are making connections and who knows where that will lead. Do the interviews but keep looking.....