Generally, all I brew are porters and stouts and the length of time required for fermentation is usually the same as for any other ale. They do, however, benefit from a longer aging process in the bottle (although mine don't usually last that long
Let us know more about the recipe you used so we can determine the level of fermentables and most importantly let us know which type of yeast you used and at what temperature you are fermenting. What I have found is that dry yeast usually begins fermenting much faster than liquid yeasts and usually finishes sooner. It can, however, be more prone to containing wild yeast strains which can lend "off-flavors." I generally prefer liquid yeast strains since there are more varieties to choose from and they can produce a product that is more true to style and cleaner in flavor. Also, temperature can affect the speed of fermentation. Lower temperature generally can slow the fermentation to a crawl if they reach below 60 degrees for some ale yeasts.
In terms of fermentation, I usually experience various levels of vigorous fermentations initially (within a few hours for dry yeasts and up to 48 hours for liquids). This usually amounts to about a bubble per second in the airlock. This usually slows after about a day or so and once it finally reaches about a bubble per minute, I transfer to a secondary fermenter (optional) in order to lift the beer from the spent yeast cells in order to develop a cleaner flavor. I keep it in the secondary for about 2 weeks before I bottle. At this point I wait at least 2 weeks to begin consuming the beer. Porter and Stouts, however, can develop a smoother flavor after a month or two of aging (once again, I can't wait that long).
What this boils down to is that depending upon the strain of yeast you used and your recipe's ingredient list, your fermentation may be proceeding exactly as it should. Let us know some more details and we may be able to help more. But above all, don't worry. Things are probably progressing just fine.