Do you have anything with your keys in your pocket that could be pressing the buttons when you sit down etc.?
The batteries should last at least a year or two, keyfobs don't use much power.
As long as you're not buying pound shop specials then you should be OK.
Even ASDA brand ones are just as good as the duracell or energiser ones.
Mine gave out after two years (both the main keyfob and the spare). Bought a pack of two (Renault badged) from the dealer for £4.44 inc VAT and so far so good after a further year (the spare keyfob has its battery still in the packet to be installed if and when needed).
Ignore at your peril. Did that for a few months but it finally refused to allow access to the car. Unfortunately the wife was driving and didn't realise Zoe has a key hidden within the card for emergency access
There are some YouTube clips on how to open up the key fob. You take the key out and use a watchmakers screwdriver to prise a corner of the seam line open and then work round until the halves separate. The stock issue battery in mine was a Sony 2025 but a 2032 seems to work just as well as it physically fits in a LEAF keyfob. A 2032 is rated at 240 mAh and a 2025 is rated 163 mAh, so a 2032 should last longer in use. Both have a shelf life of eight years.
@Mike Schooling I suppose this means one can upgrade the battery capacity of a LEAF for less than £5.