Depends on the rope and the type of splice you're doing, as well as your skill at splicing. A good estimate for double braids and cored lines is about 1.5-2' per splice plus the eye splice diameter in the case of a bigger eyesplice. .
SD, Arf145 is doing an end to end constant diameter splice for his Hood 810.
Lets see,....The first mark is two fid lengths from the end and that is the location of the crossover, so if I am thinking about this correctly you would lose approximately 4 fid lengths of line. I would figure 5 just to be safe.
I usually figure 2' for an becket splice.