You use the WatchDog timer on an AVR with the maximum prescaller to generate an interrupt once every 8 seconds and use this interrupt to wake form sleep and update a counter until it overflows past 225 (use an unsigned u_int8 register for efficiency) at which time you can write a 1 directly to the PIN bit to toggle an IO pin that is in turn connected to the input of an SCR that is wired in series with the lamp power input.
Get a 0.5 RPM motor like this...
This is a motor that will make one full rotation every 2 minutes. Look around and you can probably find one for $20. I have seen ones that plug directly into 120VAC plug, or you can get one that used 12VDC and a wall wart transformer.
Mount the motor onto a 2x4 with screws or duct tape. You can also duct table directly to a table or wall if no 2x4 is available.
Mount a switch that controls the light to the 2x4 - also using duct tape or screws.
Find another 2x4 about 1' or 2" feet long. Could also be a 1x or a broken broomstick. This will be the rotating arm.
Find 2 objects on the floor. Maybe kids' blocks, or old packs of chewing gum (they must be old enough to be hard).
Glue or duct tape the two objects to opposite sides of the rotating arm such that one turns the switch on as it passes it. The other turns it off.
Use duct tape or screws or partially chewed gum (from above step) to attach the rotating arm onto the motor shaft. drilling or poking a hole in the middle of the shaft might help.
Lots of value engineering improvements are possible depending on the atual switch configuration. If, for example, the lamp has an existing pull string switch, then you could substitute a 1 RPM motor and connect the string directly to the end of the rotation arm using a thumbtack or hair tie. The arm will now pull the string once per minute, toggling the lamp on and off.