The amount of feathering will vary depending on the size of the image, if you have, for example, an 800 pixel image (accross its longest side) and an identical but larger 4000 pixel image, then a 100 px feather will have a completely different effect on each image. So you can't really generalise as to what degree of feathering to use in any given situation.
If you have a late enough version of PS, use the 'REFINE EDGE' command instead of FEATHER (I think it was introduced in CS3). Make your selection, right click, select 'Refine Edge'. It gives you a lot of controllability over your selection and you can visually see what amount of feathering you will have before finalising it.