I mainly fish a river that supports a good hatch of March Brown, it is a SPATE river that rises and falls very quickly with rainfall. The upper reaches flows through a 2 mile band of limestone but the Remainder of the river is mainly sandstone, the bed is mostly made up of mixed sized stones and rocks. My experience of the March Brown is that the Duns appear, often in large numbers, for 15 - 20 minutes and then stop for a short period, then hatch again in large numbers and so on throughout the period of 11:00 and 15:00 hours (approximately).
The brown trout he can be switched to the nymphs, the emergers or the Duns and The method used requires close observation of the feeding pattern of the Fish.
In my experience, the hatch of The Yellow May Dun (Heptagenia sulphurea) is much more scattered, with small numbers of Duns appearing throughout the day. There are stories of the trout not liking the Yellow May Dun, but I have witnessed them being taken on many occasions.
The Large Brook Dun (Ecdyonurus torrentis) also hatches out in small numbers and some of them crawl out of the water and do not hatch at the surface. Although there is an extended season on my local river from May through to October.
The Autumn Dun (Ecdyonurus dispar) is very similar in terms of the hatch but has a shorter 'season' from July to October.