I've been waiting to hear back from Human Resources concerning a job I applied for a couple months ago (March 17th). I received a letter informing me they've received all the materials needed for my application (April 9th). How do I know how long I should wait until I hear whether they are interested in interviewing me? Should I call and ask them to tell me when they plan to make a decision? I don't know how long I should expect to wait.

Time's marching on. I'd call now to ask if they still need anything more to complete your application and find out how the interviewing is proceeding.

Don't be surprised to hear that the job has been filled. It sounds like too much time has gone by.

Yes, sounds like by now, they would have moved on. But yes I would agree, call and ask if they need any more info from you.