I've had braxton hicks since I was 15 weeks!
I wouldn't worry if you're not getting them, everyone is different. Mine have increased in frequency over the last month. When I asked my doctor about them he said the fact that I'd been getting them so long and nothing had happened in terms of me going into labour, it was a good sign that it was just normal for me. He also said they will increase in frequency and in intensity the closer it gets to the big day. He also added that real contractions are very different and I'll know when I'm in labour.
I sometimes get them every 5-10 minutes for anywhere up to an hour and sometimes only a couple a day. It just varies. My baby's activity also influences how often I get them.