I would assume this varies greatly by individual, based on biological factors. My answer would be "enough to feel fit", where moderate levels of physical play would be considered fun rather than dreaded. For me that's about 40min/day, 4x per week, of weight training. I don't know if that's typical, but having lived on both sides of the line, the benefits of feeling fit are very easily worth that lost time. Everything else I do benefits, it's akin to getting enough sleep.