The phone interview length will obviously vary from interview to interview. After a successful job application, you will often receive either an email or message indicating when the next stage of the recruitment process (i.e the phone interview) will take place (date, time etc.) and along side this the approximated time of duration. However, if this is not the case, it is common also for the interviewer conducting the phone interview to indicate at the start of the phone roughly how long the phone interview is going to last.
The length of a phone interview can depend on a range of factors, such as the purpose of the phone interview, for example -- the company hiring and the position of interest, or if it is just Human Resources checking over a few details with you, similar to that of a background check on your Resume or CV or simply a confirmation of details over the phone, then the interview is likely not t0 last that long -- maybe 10 mins or less. However, it may be that you are in for a fully fledged competency question based or technical question based phone interview which can possibly take around up to 40 minutes and perhaps more. Another dependent factor will be the actual phone interviewer themselves, some phone interviewers like to strictly keep to the time allowance, whereas others are perhaps more lenient. However, this being said the standard phone interview is said to approximately last in the region of 20 to 30 minutes, give or take.
You can also indicate how well the phone interview went from the time duration taken-- where you should take encouragement from a phone interview if you have gone over the scheduled time allowance and that if the interview was only short, then this often indicates a 'bad' sign for how it went. Which is logically sound reasoning, if a business was not interested in employing a particular candidate, they may consider themselves to be wasting resources (i.e time) by continuing on with the interview, whereas with a candidate who they were actually interested in employing, it makes sense to invest more resources to ensure that they are also interested in the job role the business is offering. This being said, it is not always the case as you can see from the article -- Short Phone Interview: Good or Bad? -- there are circumstances where a short phone interview can in fact actually be a good interview, an example being that the phone interviewer immediately identifies that they are a promising candidate and there is no need to carry on with the phone interview. There will also be circumstances where a long phone interview can in fact turn out to be a bad phone interview.
To gain a more comprehensive guide to understanding the phone interview ('screening') process as well as improving on your overall interview performance then I highly recommend that you give 'The Essential Phone Interview Handbook' by P. Bailo a good read as well as studying up on the excellent '60 Seconds and You're Hired' by R. Ryan. If you have any general remarks or experiences regarding a phone interview length, then I encourage you to leave a comment below.