Once you've completed your pre-reqs (and with your prior degrees I can't imagine you have to many pre-reqs to bang out--you definitely have all your GE, freshman English and phsyc courses done) it can take anywhere from 12 months with the aceletrated BSN programs to three years with the Entry Level MSN programs to become an RN.
Each program has different core pre-reqs that need to be completed before you apply. Typcially all programs want to see Microbiology, Inorganic Chemistry, Anatomy, Physiology. I would think with a degree in public health you may have already taken these classes. Some programs have a recency requirement whereby the core classes have to be taken within the last 10 years, however I've noticed that several programs within California have dropped this recency requirement lately.
The local CSU BSN program I have my heart set on will take two years once I start in Fall '07 (and I just feel it in my bones that I will get accepted--big smile). It's already taken me two years to finish all my prereqs, but my prior degree was unrelated and therefore I had to start from scratch with my science classes. In addition to the above pre-reqs, my program also required organic chem, nutrition and a basic stats course (this despite my three business stats courses two of which were upper division and my 15 years in marketing research, sheesh!....needless to say I got an A in Elementary Stats).
A couple of my friends have become nurses late in life--none of them without the padding of the prior degree you and I have. It took them five to seven years. They all said it was the best decision they made. Can't imagine what it was like for them (or for Studently42) having to sit through some of those boring GE classes I couldn't stand when I was 18.
So you are ahead of the game and could be a working nurse rather quickly depending on which program you get into, especially if you go the accelerated BSN route.
Of course, depending on where you live, the hard thing can be getting into a program. For me, nursing programs
within the SF Bay Area are few and highily impacted. Your prior degree opens several more avenues for you, so apply to all you can ADN, BSN, Accelerated BSN and Entry Level MSN. If you get accepted into an ADN, it would be easy for you to bridge over into a BSN program.