Many

one engineer can do it in about four hours. three and a half hours to research the ideal replacement bulb, twenty five minutes to set up the ladder safely, and five minutes to replace the bulb itself. source: I'm an engineer and I replaced a lightbulb this week.

the real problem arises when you have multiple engineers replacing a lightbulb. the time needed to decide on a replacement bulb and agree that the ladder setup is safe is quadratic in n, where n is the number of engineers. it still only takes five minutes to replace it though :)