I had a lovely Hotline Harrier or Falcon energiser, powered by a 12 volt leisure battery. Unfortunately I can't say how long they last for because mine lasted about 4 days before someone nicked it complete with the leisure battery.
I have an ancient Electric Shepherd, runs off a deep cycle battery. I brought it with me when we left the farm and that was over 26 yrs ago now. Still working well. It has outlasted many batteries. My sister has had a couple of the little energisers for short runs which have lasted no time at all.
my hotline mains fencer lasted well over 20 years,, probably more. The hotline battery one was used for around 5 years and then put away when not needed. It had been in a shed for around 15 years when we suddenly needed it. It worked immediately. Very impressed.
Current hotline mains has done around 5 years and I see no reason why it shouldn't do many more.
I have got 3 of the hotline energisers and 2 of them are over 20 years old, the other one about 8 years. The only problem I've had is the cables snapping when the plastic gets very old and brittle but they're easy to replace.
Thank you, I have mine chained to a fence post so fingers crossed it won't go walk abouts, I have a feeling my two batteries must be at the end of their lives as mine seems relatively young at 6 years old!