You record the amount of time it takes people to do a crossword puzzle or play a game of chess. After a while you'll be able to make a distribution graph of how long it takes. Then you can give an accurate estimate along with a probability.
How long does it take to solve a Rubik's cube for the first time?
How long does it take to learn to juggle 3 items? 4? 5?
How much time will it take you to measure the coast of England between Brighton and Seaton?
Usually less than 2 days, with a 90% confidence.
A different variation of the ol' how long is a piece of string question
Chess games usually have time limits, so pretty easy. Cross words depend on the size and difficulty.