I don't really know how to respond to that. We once had about 15 minutes so we played speed Resistance, where you're only allowed 30 seconds of discussion before missions, and 30 seconds for all voting stuff, and we still barely finished in time and spies won really easily because, we couldn't talk about what was going on. You need time between proposals, and especially before important proposals like the 5th one, or when a mission might be game ending, in order to try and work out what sides people might be on, question decisions people made, think about why people voted the way they did and discuss it. I'd say that on average, from M3 onwards there's about 3-5 minutes of discussion between each mission proposal, with that estimate probably increasing a bit if it's a potential game ending mission. For M1 and M2 it's a lot faster, M1 probably takes 5 minutes total or something small like that, while M2 is probably more like 5-10 minutes. An M3 end is a pretty quick game and clocks in likely close to 40 minutes (If the spies at 2-0 up on M3, at least. If it's the Resistance, you're probably getting a very quick game instead. But this is rare). M4 end is likely around an hour, while M5 end is probably closer to 75 minutes.
Now my group likes to talk and discuss what's going on a lot, but even if you were fairly brief with your discussion periods, I'd think something around 30 minutes or so would be average.