This puzzle turned out to be more difficult than what I thought it was. There was only one correct answer – from Suman Saraf – very well done!
The answer is 40 feet. There were a couple of people who sent 42 as the answer.
The trick is to flatten it out and take the path as shown in the figure, and from the sides of the right angled triangle, you can figure out the distance of the hypotenuse which is the path that the spider takes.
Hope you all enjoyed the puzzle!