lets see, a world size can be 257x257, each world tile is 16x16 region tiles, each of which is 16x16 embark tiles. minimum vanilla size is 2x2 believe, so you could fit 64 in one region tile so 64x64x64x257 = 67371008 forts in a max world, generated with no oceans of lakes and no massive mountain ranges that block embark or any other embark blocking feature.
I think you have some problems with your math there.
To convert to total number of embark tiles
(257*257)*(16*16)=16,908,544 embark squares
Total number of fortresses
(Total embark)/(2x2)=4,227,136 maximum total fortresses possible
Even assuming that your computer can load up and you can select, start, and abandon a fortress in 30 seconds, that gives for the time to make the maximum number of fortresses
4,227,136*(30/60)=2,113,568 minutes
(minutes)/60=35,226.1333 hours
(hours)/24=1,467.755555 days
(days)/365.242199 [exact days in a year] = ~
4.0185 yearsAnd that is assuming you can do it all in 30 seconds. If we figure that the average fortress will take at least 5 min from main menu to main menu (1 for the embark/abandon, 4 for finding the exact location to place the fort which will increase as time goes by) then our average time goes up by a factor of 10, giving us a time of about 40.185 years, and that is without sleep, food, or any other stops.
Edit:darn beaten to it
Edit2:I took his errors, fixed my math now