You have asked a really interesting question, and one I wrestled with when I needed a pacemaker and an ICD unit was suggested.
After reviewing my Advanced Directives and my reasons for deciding I wanted no extraordinary measures to try and start my heart if it would stop or I had a stroke, etc., I opted for just a 'simple' pacer. If I were a younger, stronger man and thought I've come back strong after a stroke/heart stoppage, I'd probably asked for a ICD. I'm over 70 years old, and suspect if my heart stopped, I'd be the worse for wear, so to speak. My conclusion was that inserting an ICD would defeat my legal Advance Directive which is on file at the local hospital and also in the hands of my loved ones.
This is probably a controversial post, but congratulations for posting it.
When my Dad was on hospice care (dying from heart failure) his ICD battery was getting low. But with CHF no one is quite sure how long the heart can last. They actually took him off hospice care for a few days, replaced his ICD with a plain pacemaker and then put him back on hospice. He did quite well with just that and eventually died from cancer a year or so later.
How long a battery lasts depends on how much it's used. Like a flashlight, if you use it all night, the batteries will run down quickly. If you only use it once a year, it will last a very long time. With average use a pacemaker/ICD tends to last about 5 years.