It depends on the species, on both your temperature and light cycle question.
Most people do a 12/12 cycle on the MVBs with most species (Savannahs, Tegus, etc.). Under no circumstances are you to leave a MVB on 24hours a day.
If your ambient temperatures are not adequate, then you'll need to switch the MVB with a nighttime bulb. As long as your temperatures don't go below 70-75 at night, you are fine sticking with the 12/12 cycle.
You can not use a thermostat of rheostat with a MVB. They have to attain a certain amount of power to start up in the first place. Also, if you have them set to turn on and off several times a day, the bulbs lifespan will be cut dramatically.
Seeing as most Monitors require basking temperatures in the 100's, you probably won't have a problem with bulb reaching a higher than desire temperature.
If I remember correctly, most MVBs need to be placed 12-16" away from the basking place. If you take them too far away, the UV isn't as effective.
The good news is; MVBs have a much longer effective lifespan than tube lighting.
Hope this helped.
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