It depends entirely on the 'bator size, how insulated it is, and room temperature. You will just have to fool around and see what gives you the proper results.
FWIW, my incubator is probably 18"x18"x2', HEAVILY insulated and in a 55 F basement... I run a 40w that is on constantly, and a 25w that is in the thermostatically-controlled socket.
But everyone's situation is different. Experiment and you'll find what's right for yours.
Honestly you will have to buy several, and just find which is right for you, I went to the dollar store and bought a 25W, a 40W, a 70W and a 100W. I ended up using the 100W, because although the 70W worked, it took a while to regain temp after opening the bator, the 100w gets back to the correct temp faster.
I made mine from a 40 qt rubbermade cooler, started with a 40 watt, which does work, but stayed on to long and made the temp flux to much. I went to a 52, and its perfect. cycles about a 3 minute interval, and the temp stays pretty stable. I use a drilled water heater thermostat and I think you need a bit more flux in temp to trip it. My fan blows right over the bulb and into the thermostat.
I made mine from a small Coleman cooler. I use a 40 wat bulb in mine. Oh and bit of advise don't buy cheap dollar store bulbs because they burn out quick. I bought some and they burned out three times while I had eggs in there. Here's a picture of mine.