In my (Tasmanian) experience March flies generally appear in January, become prevalent in February (in the right conditions, as wander points out), and actually tail off in March, when cooler weather appears - and their life cycle is complete.
The term "march flies" covers a number of different species, elsewhere known as horse flies, and generally from the Tabanidae family. There are thousands of different species world-wide. Each species has its own different requirements, seasons, habitats etc, but march flies are generally nectivorous, and play a key role in pollinating plants. Only the female flies bite, as they require blood to reproduce successfully.
Here are some that are found in Tasmania
https://sites.google.com/site/insectsoftasmaniadiptera/suborder-brachycera/tabanidae-horse-march-flies ... should revive some happy memories (not!)
cheers
Peter