I think mine is about 13 years. I wouldn't want to have a newborn baby when the first one was doing important exams like GCSEs or A levels. And then they'd have left home and I'd think do I really want to start again now I've got my independence back?
I always find it weird when people fret about age gaps! That's probably because I have only one sibling, ten and a half years younger than me. There is no normal or abnormal because each family is unique and no one knows any different.
If anything, I feel like I enjoyed the best of both worlds - I have a sibling (who, admittedly, I am much closer to now that we are both grown up) but I also enjoyed the benefits of being an only child for the first decade of my life.
I am one of those people who frets about the age gap, I wanted it to be 2 years but if I am lucky it would be more like 5. I think I am so obsessed about it because my DB is 2.5 years younger than me and we are close and I really want my DD to experience that. Also, have been TTC for 2.7 years so I guess trying for so long means you are more likely to become obsessed!