Mythbusters did a fairly scientific study of this.
IIRC the conclusions were .........
Traditional light bulbs - if you are out of the room for more than 0.36 seconds then it is worthwhile turning the light off. They took into account reduced lifespan.
Fluorescent tubes - because of higher starting current the time was about 20 seconds.
Can't remember results for modern bulbs, but IIRC it was similar to traditional bulbs.
The overall conclusion of the experiment was to switch off lights whenever you leave a room. Reduced lifespan is negligible compared to electric saved.
what they said
wear your night vis goggles for a low energy life
frequent turning on and off reduces the life span of a Compact Fluorescent bulb quite a lot. The lifespans quoted are based on normal use.
In toilets etc they last loads shorter and quickly lose their full illuminating power (go dim) !
This can be a concern if you are concerned about increased mercury use or the fact that the bulbs are mainly manufactured then transported from china.
If you ensure you leave 20 minutes between switching off and on or vice versa and limit yourself to 30 on off cycles a day their should be no concern.
With higher tech more expensive lamps you can switch more frequently
With LEDs you don't need to worry the same.