There are 24 hours in a day. For the sake of discussion (and to simplify the math), lets assume you have the lights up for 41 days, and keep them on all the time. That is (24*41) hours, or 1000 hours; that is why I went with 41 days.
Assuming a string of lights draws 5 watts (the draw of one string of 70 LED lights), keeping the lights on 24/7 for 41 days will be 10 Kilowatt-hours. At $0.12/kWh, that’s $1.20 to light one string for longer than the holiday season lasts.
Since the season is shorter and you typically keep the lights off during the day, your bill will be far smaller. Lets first go with 6 hrs of light a day and up it to 40 string. Not four, forty; tens times the lights Enough to wrap the porch and then some. Twelve dollars for nearly a month and a half.
Maybe this will give you a better idea though. They assume 5hrs/day and a 30-day season. If you are the type that puts up the animated displays with music and all, the sort of displays that get YouTube videos, then you’ll still be under $50/month if you use LED lights. Old-school incandescent lights will cost 5–10x as much to run though, which means that an average house with 12 strings of lights and a couple lawn ornaments will spend ~$11/season instead of ~$1.50/season.
TL;DR – Unless you have the type of lights that would but the Vegas Strip to shame, you spent more money changing your lights than you will save on your electric bill.