Night Citiscapes - Neon/Flood-lit Buildings

The best time to commence photographing neon or flood-lit buildings is when the ambient, or daylight levels are similar to the artificially lit buildings themselves. This, generally, is at a point after sunset, when all the lights have been turned on, but there is still a bit of light left in the sky in order for you to record some detail in the unlit areas of the buildings and to show some colour in the background sky.

If you were to start photographing any earlier than this time, the ambient light would be too bright for the lights on the buildings to have a proper effect, and anytime later, the buildings would probably come out OK, but the surrounding sky and unlit parts of the buildings and other areas would turn out black.
If the building itself fills the scene, you'll find there is no need to alter your camera's exposure settings, because the overall lighting on and in the building should be fairly even, as exposures go. Unless, of course, you have any bright lights in the scene, that are pointing directly at you, then you may have to use some manual control.
In this case, simply switch your camera to aperture priority mode and set it from f11 to f16. Why? Remember 'depth of field'? Either of these settings will ensure you have plenty. However, we are shooting at night and your shutter speed is going to be quite lengthy, so a tripod is essential.
On the other hand, if you are photographing your buildings from a distance, there is a good chance you will have a lot of black areas in the scene, as a result, your camera may tend to want to overexpose for these areas and your buildings will turn out decidedly lighter than you anticipated. Probably your best option here would be to use your camera's 'exposure compensation facility' if it has one, and take one shot at "minus 1" compensation, another at "minus 2" and another at "minus 3". At least one of these exposures should give you a reasonable result. Otherwise, if you can take a separate meter reading of the building itself, without the surrounding blackness and set your camera to that reading, that should also give a good result.
This exercise could also apply to homes lit up with Christmas lighting, or flood-lit trees, or even monuments, or statues.