We started the 'A Vested Interest' series in 2007 and it took over a year before I came up with an ending we were happy with. At 170,000 words A Vested Interest was too long though for a printed book. We cut it heavily but still ended with a 140,000 word book. There was no alternative, we had to split it into a two book series. Doing that, we thought, would allow us to put back some of the content we had cut and expand the second book (Dark Secrets) a little.
Well that was the plan. We ended up splitting the second book and making a trilogy by adding 'No Secrets'. The original ending didn't quite fit now so we moved it into a fourth book - Stones, Stars and Solutions.
And so it goes on. We are now writing book 10 and 11 of the series. Shelia has written a spin-off 'Blood of the Rainbow' trilogy. Altogether it's 2 million words so far! In terms of time, we've only covered a few months. There is an end in sight but not for another 5,000 years. Maybe I'll get to use my original ending then?

About the Authors
John and Shelia Chapman are a husband and wife team who met on Internet and crossed the Atlantic to be together. John, an English ex-science and computer teacher contributed the technology and 'nasty' bits while Shelia drew on her medical experience in the USA and produced the romance. The humour? That came from real life.