Answer: Most fixed voltage devices work at a standard 3.7 volts output. This does vary by model, manufacturer, and also battery charge (in the case of mechanical mods or non-regulated types).
The big plus to VV is that you can tune your vape a bit by changing the voltage. Always start on the lower end and slowly increase then decrease until you find a sweet spot for a particular juice. Some juices taste better a little cooler, and most taste pretty good at or around 3.7 volts, other juices taste better a little higher voltage.
They'll all be pretty good and acceptable on a fixed voltage device, but you obviously can't change it to get the better taste for some juices over others.
Technical note: If you know the resistance of your attachment (say 1.8 ohms for instance) then if you multiply that by 2 = 3.6 volts. Plus or minus a few fractions of a volt is a pretty safe range to run most things. Or you can just go by taste, which is pretty much what I do. I alter my voltage or wattage for a hotter or cooler vape depending on my mood or the juice or any number of reasons.