I think it is very dependent on individual circumstance. It FEELS like we should be 'happy' living on 35K/year, because we were mostly happy living on that in college/grad school. Of course, in grad school, we were
1) in a fantastic location with tons of outdoor activities,
2) we were INSANELY busy so we didn't do a lot of 'bored' spending,
3) we had few assets, so didn't have to worry about ongoing costs to maintain a house, etc.
4) our cars were beaters (sure, they broke down regularly, we despised them, and they caused us unending stress, but they were ultra cheap)
5) we were in the bloom of youth and had almost no health care needs,
6) we ate a lot of nutrient poor cheap food, and
7) we lived for almost ten years in a crammed studio apt, which was incredibly cheap.
Now in middle age, we
1) have lived for years in a city and region that sucks balls so it costs us travel money to do almost anything we really love
2) the city is cheap, but we sure don't want to stay here in retirement (see 1), so we have to anticipate higher rather than lower housing costs in retirement, even if we downsize from a 3/2/2 house
3) we work constantly and almost never recreate. If we had more free time, we'd spend more money
4) we drive a better more expensive car, and own two houses (we need extra house because of supporting a relative, and that could continue at least 15 more years)
5) our health care costs are now a real expense, and could very likely go much higher over time
6) we eat more expensive, nutrient dense food
7) we are helping to support family members (adds about 10K/yr of cost)
We currently spend 50-60K/year, spread across two households. So we anticipate needing at least that much to get by comfortably in retirement, but we expect to need more than that if we want housing in a better location, fun money, and most importantly, cash for health care costs as they increase.
So we are aiming for 70-80K/year income minimum. Pretty straightforward math for our situation, seems to me.