We ride about an hour each night, usually 8-10 miles. Trying not to put too much money into this since we'll only use them the month of November. Once the slopes open in late Nov-early Dec, I'm in boots teaching 7 days a week til mid April, so the bike is out of commision. By April, daylight savings kicks in and I have light to ride in the evenings.
Trying not to spend ~$75 for a light, times 2 bikes (mine and the wife) that's $150+ for about 4 weeks of use a year. Maybe we'll have to just stop riding except for weekend single track once daylight savings stops next weekend.