The Question Browser provides question statistics that you can use when deciding which questions you want to use. These statistics are based on all students who have answered a question since the last time the question was updated.
Indicator  Description 

Question Usage 
Indicates how many students have answered at least part of the question since it was last saved.

Difficulty Index 
Indicates the percentage of students who submitted an incorrect answer on their first try for any question part since the question was last saved.

Time Estimate 
Indicates the median time in minutes that students have spent to answer the question since it was last saved.

The Fine Print About Time Estimates
Time estimates for questions are calculated using data from assignments set to question part or question level submission. All of a student's submissions for a question, or question part, are counted towards the time it took the student to answer the question. The time spent on a question is measured starting with the student's last interaction with an assignment and ending with the time the student submits an answer. Interactions with an assignment can include:
 downloading an assignment
 entering answers
 submitting answers
So, the time spent between downloading an assignment and submitting an answer to a question would count as the time spent on that question. The time spent between submitting the previous question and submitting an answer for the next question, would count as time spent working on the next question. When a student submits the answers to several questions at once, the time spent on the submission is divided equally between all question parts in the submission and the time estimate for each question is the sum of a question's parts.
The time between a student's answer being saved by WebAssign and the student submitting an answer is calculated into the total assignment time.
For example, if a student uses 3 submissions for a question the sum of the time spent on each submission is added together to calculate the amount of time a student spent on a question. This time is then averaged in with the times for other students to calculate the question time estimate.
The estimated amount of time it takes to answer questions is then used to estimate the amount of time students will spend on an assignment and the estimated range of time students will spend answering the selected number of questions in a question pool.
 Question Pool Estimates

The time estimate for a question pool is a range that is calculated using the sum of the lowest and highest time estimates for the number of questions you choose to use from the pool. For example, if you choose to use 2 out of 5 questions in a pool, then the lowest time is the total of the 2 shortest question times and the highest time is the total of the 2 longest question times. A good question pool will have a small range because all questions in the pool should have similar time estimates in order to ensure fairness to all students.
 Assignment Estimates

The time estimate for an assignment is calculated by adding the estimates for all of the questions in the assignment with the average of any question pool ranges in the assignment. When calculating the time for an assignment that contains a question pool, the average of the question pool's range is used. Any questions without time estimates are not counted toward the assignment time estimate and should be manually accounted for.