When a Mayfly board is asleep, it draws around 0.27mA. If you have a microSD card in the socket, the card draws a little current on its own, so then the Mayfly draws around 0.43mA when sleeping with a card.
When the Mayfly is awake and idle, it draws 6.5mA.
Battery life in a deployment would depend on the size of the battery and what sensors or other devices you’re powering with the Mayfly. You can do some estimations at this site: Oregon Embedded Battery Life Calculator. Just plug in the above numbers for the Mayfly, your battery capacity, and information about the waking time.