It varied episode to episode. I'd usually end up putting 10-15 hours into research and writing but with some topics I couldn't tackle in 7 days, I'd have to slap a roman numeral after it and try to finish it next week. Or, come up with some sort of cop-out for the week entirely.
In one case [1] I ended up with a teaser + 3 parter that wasn't super congruent because research for part 3 sort of changed how I would have approached the script for the previous two.