Used to box Golden Gloves and although being a fossil now, I still shadow box with 3 pound weights, and have a heavy and speed bag that I use. Along with pushups, situps, side and back raises, I spend about an hour a day total 4 or 5 times a week.
15 min shadow boxing; 15 minutes heavy bag, 15 speed bag and the calisthenics which take approx 15 minutes.
All the above are done non stop, rather than 3 minutes and rest a minute.
So, in a real fight I'd last about 3 minutes before I fell flat on my face from exhaustion.