How long is ok on preds?

How long is it ok to take preds?  I've been on them at varying doses since 2003, with one break for a few months.  The lowest dose when on them was 10mg, highest was 40mg as far as I can recall.  I've been on them continually for over two years now.  I've had bone density tests and I've been assured everything's fine.  I did try a special non absorbent type a month ago which didn't seem to work for me, so changed back two weeks ago.  I'm still having a flare up with loads of blood and mucus numerous times a day and severe cramping.  My doctor says it's ok to be on them, he's like to get me off them but everything he's tried hasn't worked.