Its easy to count if you think about it.
1. You render region to see how many passes you need to get a clear image at the region you chose.
2. You then press render of the image and see the first 1-2 passes that shoud not thake longer than like 4-5 min, so if your first pass takes 1:20 and second 1:24~ you can estimate, that your 100 passes will take 1:22x100 thats about 2,5hour~