I played around with long exposures earlier this summer. >2hr exposures were not a problem, save for the battery running out. Here are my notes:
camera tripod mounted with eyepiece cover & remote trigger
full manual, bulb setting, autofocus off (to save battery, but getting sharp focus in dark was tricky)
long exposure NR on or off (I was trying to catch meteor trails and wasn't sure if they'd be cleaned out; otoh, fireflies added trails of their own to some shots)
exposure depends on the lens you're using. Here's an example with the 35 mm prime, 24 min @ f 5.6, iso100
The biggest thing I found was trying to keep light down. If you want good long trails, you need time. Any light from the moon, passing cars, nearby cities, etc. washes out your pic and makes your background an unsatisfying brown. Better hope the clouds don't roll in. Planes and satellites can give you odd streaks too (not sure if NR would help there).
edit- gah! cruddy jpg artifact not visible in original. star colors muted too.