Speaking from experience, it cost mW about a week or two per year to maintain GitLab for the startup I worked at.

My personal GitLab on the other hand really does take only a day or two per year.

That said, a week or two per year is just what it costs to maintain any one thing period. I spent about that much time maintaining PCs in the office, or my personal proxmox setup. It's not onerous at all.

GitLab is super bloated and a little sucky to admin, but it's not too bad all things considered. I'm admin in my new job's GitHub org and it sucks a whole lot more to maintain.