As discussed in a question about teaching mental arithmetic, my fourth grade math class included a daily 3-minute exercise with 50 - 100 problems. Each day's problems were either addition (of one- or two-digit whole numbers), subtraction (of one- or two-digit whole numbers), multiplication (of integers between 0 and 12), or division (of integers between 0 and 144 by integers between 0 and 12, yielding integers between 0 and 12).
If the students have been through such a program to encourage them to memorize basic math facts and perform arithmetic quickly, it is therefore reasonable to expect students age 11 or higher to correctly answer items 1, 3, and 4 in 2 - 10 seconds per question, with an error rate on the order of one percent. Question 2 might require more steps, such as writing out the problem, and two different carries. Thus, question 2 might take 5 - 60 seconds.
I would not be surprised if the time to manually perform addition or subtraction is proportional to the total number of digits involved. Similarly, I would not be surprised if the time to manually perform multiplication is proportional to the product of the number of digits involved, multiplied by a different proportionality constant. I expect long division to take even longer than the corresponding multiplication problem.