Depends on the wattage, but they're tons cooler than any incandescent or HID lamp.

Generally, for CFLs, it's something like .3 CFM multiplied by the number of lighting watts to cool them within a few degrees of room temperature.

I ran a box with 6 26 watt bulbs that was about 2.5 cubic feet. I used a 8 inch desk fan and it kept it more than cool. I can't remember what the cfm of it was, but the fan only cost like $7 so it was far from top of the line. When I first ran the box I let it go for about 12 hours with no fan on and it got warm, but no where near starting a fire.