I suppose a very stupid question for experienced FCP users, but I simply can't find the solution.
My Sequence is 2min30sec long, but when I export, the movie is almost 3min12sec long, with black screen as from 2min30, up to the end.
By the way, I'm still using FCP 7, but I guess the solution for my issue will be "universal".
How can I make sure my exported movie is only 2min30?
It sounds like you may have made an error in marking the end point of the timeline. Go to the timeline's end, and mark an in-point ("I") where you want the thing to end, then hit the right-arrow or the "media end" button to take you to the end of that long section of black, mark the out-point there ("O"). Cut or delete the highlighted section.
Now, re-mark the in and out points of the part you wanted to output, save the project, then export.
This kind of repair can also be done from within Quicktime or using VLC or MPEG Streamclip to mark the section you want to export.
this didn't work properly but I managed to find a solution. I think there was a bug in the sequence, because I copy-pasted all the clips into a new sequence, then simply indicated an "out" point at the end of last clip, exported and this worked fine.