The typical maximum power that can be pulled from your socket is 1800 watts during turnon of a grid connected device. If your CFL is 20 Watts continuous then for energy consumption your breakeven time would be 1/90 of the turn on time or < 2 minutes.
A more serious problem is how much off / on /off / on cycles impact the lifetime of your CFL. I would guess that 10 - 15 minutes of off time would be a winner overall.
If you really want to settle further energy questions, get an inexpensive watt/energy meter such as the Kill-A-Watt available for $26 + shipping from
http://store.solar-electric.com/kiacpomome.html
You can also download the manual from this site.
An incandescent bulb has a cold resistance of about 10% of its operating resistance so it pulls about 10 x its rated power during warm up.
Note that the turn on transient put much higher stress on the filament than just operating. (Don't most incandescent bulbs burn out during turn-on?)
regards, EnergyFred