For me, 4-6 hours normally. By the time the campaign is on session 7+, I've got the major BBEGs worked out along with enough of the NPCs' stats for them to be playable. I've also fleshed out the PC plotlines and subplots, the campaign plots and subplots, and the various side-events so that they can run themselves with occasional updates from me.
The 4-6 hours each week is usually spent 1) Going over the DM's version of the character sheets and noting where I expect any curses, diseases, items, plots, subplots, etc. to intersect the PCs in the session; 2) Reviewing last session notes so I don't accidentally drop important details - like the BBEG's henchman getting killed before the final showdown (love/hate it when that happens, the clever <censored>); 3) Updating notes for the next session to account for #1 & #2; 4) running an eyeball over the updated next session notes and beyond, so I've got them all fresh in memory and can wing it if needed.
Occasionally it's much less - if the players are being overly paranoid they may not get as far as I expect. Other times, it's much more - like the time the PC's actions resulted in a complete paradigm shift in the campaign, causing me to call an early halt to the night and spend all my free time over the next two weeks working out the results.
The work is fun - I get to draw on the players' creative input, add it to my own, spice it with some inspiration, and add plot thickeners. My biggest reward in it is seeing how the players play with the results, and how much fun they have with it. I don't enjoy working out feat chains or skill point assignments, among other things. Fortunately, others on ENWorld have given their advice on how to deal with that, so it isn't a pain anymore.