Around 10 years. I think they met at the end of uni and I was born when my mum was 30 or so.
My parents had the sense to a) enjoy their youths and go off doing all kinds of **** that a child would get in the way of and b) get themselves in a decent position financially so that they could support children as best as possible. (also, they had the sense not to go breeding like rabbits and limited it to two of us, yay vasectomies)
They'd been together for between 3 and 4 years, my mum was 36 or thereabouts and my dad was 38. It was a year and a half later for my brother. They married when I was 6 and divorced when I was 14. I think if I hadn't been conceived they wouldn't have been together that long but my mum had been told she couldn't have kids so I guess they weren't planning on becoming a family. I'm guessing having kids kind of trapped them together if anything because they both have strong views on family and appearances. I can't ever remember them being happy together and the years leading up to their divorce were exceedingly hostile.
They both now have their own long term partners (6 years and 6 and a half years).