I would suggest checking with other members of your IRB committee for your particular hospital to find out the time commitment as they may be able to give you the best estimate of time based on the needs of the IRBs submitted. i.e. 4-8 hrs per week, 8-16 hrs per week, 20 hrs / month requirement, or whatever your hospital requires, etc. This time commitment may also depend on how many individuals are on your IRB committee in your hospital as to how much of a time commitment is required. Is your hospital IRB committee is just looking for additional members to assure that they have enough individuals for committee participation and to assure they can make a forum when reviewing IRBs? Just a thought.
The time it takes to serving on a hospital IRB committee depends on how much research that hospital does. For example, If the hospital is a teaching hospital and conducts a number of research studies, then, serving could take a considerable amount of time as then the IRB committee may be reviewing a number of IRB applications. If the hospital doesn't do that much research, then there shouldn't be that many IRBs submitted. You might also check with the hospital's research dept and/or the IRB committee to find out the data regarding the number of IRB applications submitted the prior year to the IRB committee to give your an indication as to how many IRBs were reviewed previously. As It is difficult to give a specific amount of time serving on an IRB committee as it would depend on the number of IRBs submitted for review. Reviewing an IRB does take time. Each IRB (whether from a hospital or university, has a very detailed application process), for the applicant (in order to reduce risks for the hospital or university.
The job of the hospital or university IRB committee is to review the IRB application to assure that the IRB submitted meets with the hospital's and/or university's strict standards in order to not only reduce risks to the hospital or university, but also assure that any potential human subjects are protected against any risks from the research experiment. This IRB process can take anywhere from 2-6 weeks depending on the IRB committee's workflow and the number of IRBs, but this certainly doesn't mean that the entire IRB committee meets the entire time during the 2-6 week timeframe.
Once you find out how many IRBs were completed the prior year, then perhaps you can ask ahead of time for your hospital's IRB workflow in order to confirm the timeframe regarding IRB committee time commitment. Evaluating the number of IRBs from the prior year based on the average timeframe per IRB may then give you a better indication of the actual time commitment for your particular IRB committee, if your IRB committee is unable to give you an exact time commitment directly.