I do admit, I often do crazy amounts of prep for my campaigns. And I didn't even show all the random tables, and custom weapon/item lists, weather tracking, etc.
Overall campaign: quite a bit of prep ahead of time - complete rules overhaul, projected storyboard for what I'd like to run (and may get to actually run bits of), and worldbuilding: maps, history, major NPCs, weather and climate, realms and nations, etc.-How do you prep your campaigns?
If I'm using a homebrew adventure I'll have made enough notes and maps ahead of time to run it; if I'm using a canned module then I do very little beforehand. I improvise more than my players probably realize, and have to when they left-turn - which in fairness they don't do nearly as often as they might.-How much do you write before hand, and how much do you improvize?
DM side: paper, writing implements, module (if relevant), notes from previous sessions/adventures, DM screen with lots of sticky notes, smartphone; and right nearby are rulebooks, minis, maps, tables and charts, etc.-What is on your table during a session?
4 hours, give or take; once a week, give or take.-And how long do your sessions generally last?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.