What wattage are the bulbs? Are you using 120V 15A receptacles? You should never plug more than 1500W worth of anything into a single outlet, but you should be able to safely plug 1500W worth into a single outlet without creating a fire hazard. If they're 100W lights, that's 15 strings. Bear in mind what else is on that string, since loading up a receptacle like that means you can have very little else on any other receptacle fed from that breaker.
Watch your extension cords, too- those cheap thin ones can't take the load of that many lights! Nor can most christmas light timers. You'll have to check the label on your lights, too, for the limit on daisy-chaining them; IIRC, the lights we have include 5A fuses and can handle 6 in-line, but it will vary from light to light.
That said, I metered a string of 100 cheap lights last year and found they only drew about 0.3A a string, which surprised me greatly as I was expecting closer to 1A/string. I'll have to test it again this year when we drag the lights out again.
Are you using 120V 15A receptacles? You should never plug more than 1500W worth of anything into a single outlet, but you should be able to safely plug 1500W worth into a single outlet without creating a fire hazard.
Why 1500 Watts? Why not 1800, why not 1440 Watts? Is this figure just pulled out of thin air?
(BTW, Tex is listed in his profile as an Electrical Engineer.)
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.
Upon further reflection, I was slipping into talking-to-a-layman mode and using a rule-of-thumb estimate; 15A is not 1500W. You're right: 1440W if they're on for longer than 3 hours, 1800W if they're only on briefly.