kkara, you can get away with about 25 sec exposures before field rotation starts to really bite and stars become too bloated from uncorrected random mount movement. I find 30 sec is pushing it a little too far and 15-20 sec is a bit underexposed.
Here are some examples of what I did on my alz-az mount with my 8SE.
http://www.astrobin.com/users/Amaranthus/
I doubt I'll be doing any more on this setup though, as I've now got my AZ-EQ6 mount up and running!