Anywhere from 10 minutes to an hour. It all depends on the recipe, how much time I have, and how much energy I have that day. I typically plan out 5 meals at a time and shop for them, then choose which to make on which day depending on circumstances and what I'm in the mood for.
I don't usually make mashed potatoes because it's a relatively nutrient-poor, high-calorie side dish, and it's not worth the time to me. It's not that I don't spend that much time making food, ever; it's that I don't want to spend that much time on something that has almost no redeeming qualities (except deliciousness, so of course I make an exception when that makes it worth it), especially when I have other food to make that is occupying my time, energy, and attention.