I can give you my own experience if it helps. I have my fukushidoin certificate, which I got by recommendation through my teacher under Shibata Sensei. I was already a nidan of two years at that time, and had been asked to test for the certificate under Chiba Sensei, but our dojo split with Shibata and didn't get the chance. When I decided to open a dojo, I went ER and simple protocol was that I have my certificate reinstated with the ER.
I would not have felt comfortable starting a dojo without this certificate, as I was only a nidan when the dojo opened (now sandan). It isn't required for having a dojo, but is required to test your students.
The length of time it takes you to be able to be proficient enough to start a dojo is really quite individual. Going into aikido with the idea that you want to be a teacher is quite premature IMHO. I've been practicing regularly since 1982(3) and only started teaching in 1994(5). I didn't open a dojo until 2002. When I started, teaching was about the furthest thing from my mind. It has just been a natural progression of my practice.
Both Jorge and Peter R. are teachers and can talk about their experiences too to help give you some perspective on the time and commitment required to do this.