Yeah it's an unhelpful answer to a poll, but I imagine most people that DM most of the time would say that it really depends on each individual DM's own style and needs.
As far as players are concerned, I can't imagine that they know how much their DM preps unless the DM tells them. I know that when I was only playing I always assumed my DMs prepped a whole bunch- if they had told me they were doing it all off the cuff I'd have been amazed... I guess if you enjoy the game, you're assuming they put a lot of work into it? At least, I would have.
Another important note caveat to this question is "online or in-person?" In person I require basically no prep, except to pull out a bunch of minis that I think might be relevant to the location that the party is in or could reach. I am very improv-happy, reacting and coming up with appropriate things on the spot and using tables etc.
Online I need to spend at least a couple hours prepping for a rough location, all technical stuff to make sure that the quality of the online session meets my standards- walls for line of sight, making sure I have the statblocks imported into the game for creatures, finding art for creatures and locations, etc etc etc.