Time lights come on in Vegas
 

Planning on an anniversary dinner at Top of the World & wanted to watch the lights come on while there. Need to make reservations soon. Thanks.

I think the "when will you be in Vegas?" question is important. You could call the restaurant and ask, if it's very soon.

Like trippinkpi eluded to, the time the lights come on probably varies throughout the year depending on the time of sunset. On my last trip to Vegas, I remember the lights started coming on close to an hour before sunset, and due to the sunlight, they aren't that impressive until it starts to get dark. You'd probably be best to try to get reservations for just before sunset. That way you'll see the before, during and after effects. I had dinner at the Top of the World a few during my last trip and it took around 2 hours for just 2 people. The following link will show you sunrise and sunset times for various times of the year. http://aa.usno.navy.mil/data/docs/RS_OneDay.html
Its from the US Naval Observatory, so it's probably pretty accurate.

Pretty much when the sun goes down and it starts to get dark.

I don't think the lights ever go off...do they?