Mic,
Everything really depends on your goals and experience. When I started running a long run was anything over 30 minutes! I don't run marathons or even half-marathons (yet), but I do run a few 5k's and 10k's each year.
Over winter I am in what I consider maintenance mode and I only run about 3 times a week - My long run is about 6 miles (or around an hour) and the other 2x are around 40 minutes. I just recently acquired Imax and Imax2 and I have been using these once a week for my intervals in addition to my running.
In early spring, I will begin to increase my time/distance until I am running 5-6 times a week. This summer my long run was around 9-10 miles (or aound 1 1/2 hours), 2x a week runs were 6 miles (1 hour), 1x was 1/2 hour interval and the other (if I could fit in a 6th day) was just 1/2 hour run.
There a lot of more experienced runners on this board who run a lot farther or more often than I do and who have different training goals than mine. Their long runs will be much different than mine.
The really important thing to avoid injury is to S-L-O-W-L-Y increase your time/distance and not try to add too many things at once. The rule of thumb as mentioned in an earlier thread is no more than 10% increase per week.