I have a loaded 2015 MacBook Pro. My wife insisted on it - we felt flush at the time. It spends 6 months unused, and six months as a desktop, plugged in. But if you go to use it mobiley, the battery life is about 40 minutes.
I got 4-6 hours on my 2015 MBP depending on what I was doing after two years.
You could get a program like iStat Menus to keep an easy eye on your CPU to see if something is using it. I think it's required in 2020 where Spotify, Discord, Slack, Steam, a browser tab you opened last week, etc. will regularly decide to start eating a whole core in the background.