To put it in perspective, let's assume 10W lights (there are stronger and weaker ones, but it's a good number for a strong LED bulb).
If you leave 5 of them on unnecessarily, for 5 hours a day, that's 0.25 kWh. Repeat for 365 days a year, and it's 91 kWh.
If you live in Germany (notorious for high electricity prices), that'll be... about 40 EUR per year.