My company does some holiday lighting at some HOAs, and Steve's right, we calculate at 0.3Amps per strand, and the inline fuses vary by manufacturer. You'll find 3A and 5A, and maybe even a few other values. This will define your strand multiplier and overall length.
If you're unsure what your draw-per-strand amperage is, hook 2 or more up, slap on an ammeter, and devide the total amperage by the number of strands to get an average (Amps/# of strands=amps per strand).
Again, as Steve stated, careful with your extension cords and multi-oulet splitters. This is usually where it gets ugly. I don't actually do the holiday lighting installs, but I get to go out and fix it when those guys screw up. I've seen quite a few melted cord-caps.
And please, use GFCI protection. Kids tend to be curious about anything that makes pretty colors.
Good Luck!
