The questions from Stargazer keep on coming back from time to time on various subtitling forums. I was asking myself the same questions few months ago, after having translated my first few subtitles! So in addition to what SmallBro stated, I would add the following:
If you look at the guidances and rules which exist over the Internet for the professional 'subtitlers', these are aimed at the whole syndication industry to cope with the widest panels of subtitles readers. The '40 characters per line' rule ensures that various TV channels could put characters big enough for short-sighted people to be able to read on their 21" 4/3 television. Some TV channels will even go down to 38 or 36, to superimpose the text over a black box background (good reason for having the minimum text on the first line).
But do these rules really apply to fansub website users, most of them being likely to watch movies on >32" 16/9 LCD TVs without sight impairment?
I personally prefer subs with a maximum of 50 characters per line / 100 characters per page, which allow a better splitting of sentences at commas or natural 'breathes', without the need to add those annoying starting or ending '...'. So the choice is yours, you decide to which panel you wish to dedicate your subs.
More important than the number of characters per line, is the reading speed. The English syndication rule is around 16 cps (characters per second), again to cope with the widest panel. As you are interested in translating in Swedish, have a look at the book
http://books.google.fr/books?id=KOo7mfiXibUC : the expected reading speed for Swedish subtitles was 7-10 cps in the 80's, and increased to 9-14 cps in the 2000's (page 133).
So again, does this apply to fansub website users? If you use the 16 cps rule, you will end up doing the same as the professional translators: savagely cutting into the dialogs, and showing only half of these. I think the vast majority will prefer to have almost verbatim translations. Having done a small analysis of the addic7ed and OS fan subs, I now have personally put the limit for my English or French subs to 25 cps, with a strict limit to 35 cps when offset goes over three lines (meaning, I prefer a large time offset or a cut translation than a sub I don't have time to read!). I also set a minimum of 500 ms/line.
You could do this useful exercise: for instance HIMYM S08E01
http://www.opensubtitles.org/fr/subtitles/4670280/ has a very fast one-minute length monologue at 11'43'', with some parts of the verbatim transcript as high as 50 cps. Translate this part in Swedish, look at the episode, and see what speed you are comfortable with. Like that, you will be able to set your own limit over which you will have to cut into dialogs without remorse...