I have worked in fashion and been a fashion expert, or guru, for over twenty years now. People often come to me for advice on current trends and also ask me to predict future trends. Whilst I cant predict the future I can give you a very good idea of what to expect and when it comes to combat boots I can guarantee you that these will be staying in fashion for a long time to come. This also goes for the other similar boots such as military boots for women too!
One of the main reasons I believe this is because the boots themselves are built to be tough and built to last. I often find that a fashion trend lasts for as long as the item of clothing itself lasts. That is why the cheaper and more gimmicky trends don't last long as the items are often worn out after a few washes and people aren't stupid enough to rush out and buy more. With combat boots its different since these boots are expected to last the average person 4-5 years!
So I noticed that combat boots have come in trend in a really big way in the past year due to many celebrities wearing them and since the average pair will last so long you can expect this fashion trend to last for a long time too. Its hard to put an exact time scale on it because as long as people are wearing combat boots it will influence other people to conform to the fashion trend and so this might well be a trend which is here to stay.
It certainly wouldn't surprise me if combat boots remained in fashion for the next decade or so. At least until the economic climate changes as that might influence people to try new fashion trends, however for now its here to stay!