My fuel pump buzzed for several months then stopped buzzing. A few months later, I started having fuel delivery problems, most notably when accelerating. Almost immediately, I had problems starting it and soon after that, I couldn't start it at all.
When put in that context, it seems rather obvious that I had a questionable fuel pump (this car has two pumps). However, it wasn't so obvious to me at the time. I had forgotten about the fuel pump buzzing. A couple of weeks of troubleshooting here and there and I still didn't find the source, although I did discover that my EHA was leaking and that the rubber hoses in and around my fuel pump assembly were hard and cracking. So I replaced all of those and my fuel filter.
Anyway, back to the main point. It very well could continue to work for awhile. However, I would not take the car on a long trip unless I had a spare fuel pump and the tools to replace it.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino