amy! asked how I figure out the number of hours a project will take. She's going to hate my answer. You see, she's really precise when it comes to numbers...it's that computer science degree and all. I, otoh, love imaginary numbers. The biggest imaginary number of all is the one I come up with for how long it will take me to stitch a project. (Please note: I know that imaginary numbers exist, are real, and have specific meaning. I love language and I'm playing with it here. If that bothers you, well, I don't give a flying fuck.)
Don't get me wrong. My equation involves real numbers and real math like we learned in school. The imagination part comes in at the precise moment where you simply have to believe the math comes out right; that is, that the actual number of hours and the calculated number of hours will have anything to do with each other.
I got the "equation," iirc, from Deanna West of The Needleworker. Take the stitch height, multiply it by the stitch width. Divide by 100, or the number of stitches you can do in an hour. I stick with 100. I have no idea if that's really the number of stitches I take in an hour. It sounds good, though. Voila!
Here's an example: Sheepish Designs has a pattern called "Spotless Innocence." It is 220 stitches high and 146 stitches wide. You do 98 stitches per hour. 220*146=32120 (estimated number of stitches). 32120/98=327.75 hours.
If you really want to be precise, you would have to know the actual number of stitches in a given project. There are designers who will tell you this. For example, on Another Martini the designer gave the actual number of stitches (who wants to count?). So I know it should take me about 15 hours.
Several factors will affect your how "real" your number is: how many color changes are there? Do you have to futz about mixing 2 strands of 930 with one strand of 932? Are you stitching with one strand, or do you have to strip 3-6? Is the design packed full of stitches, lots of blank in the middle, or are you doing an alphabet sampler?
See this is why I hate Martha Beth Lewis's articles in Just Cross-stitch. She'd have you counting the exact number of stitches, and would have sub-functions for stripping floss, numbers of flosses used, blended needles, and how many breaks you took to eat chocolate.
Hey, kids, it's not computer science! ;)