How long will this low duty 555 LED blinker last on batteries?
The 555 timer itself will use a substantial amount of current no matter how little light is consumed by the LEDs.  There exists a lower-power almost-equivalent chip called a 7555 which might be suitable for your purposes (if you found a correct circuit for the 555), but if you use it you should probably increase the "timing" resistors by a factor of ten to a hundred compared with what you would use with a 555, and reduce the capacitor likewise [increasing the timing resistors by a certain factor and decreasing the capacitors by a like amount will reduce power consumption, but may make the circuit's behavior vary more with things like supply voltage; so long as the resistors are large enough that overall power consumption is dominated by the LEDs, little will be gained by increasing them even further].

An alternative approach would be to use a small microcontroller to operate the LEDs.  If you want to have the LEDs on for e.g. a half a second every ten seconds, the power used by 7555 the and attached resistors would be less than that required by the LEDs, but if e.g. you wanted to be on for 1/20 second every five minutes, more power would be spent on the 7555 than the LEDs; a microcontroller could cut power consumption tenfold.

Once a circuit is decided upon, figure out what portion of the time the LED is on, determine the current required when the LED is on and when it is off, and use those figures to compute the average current.  Then find out the number of milliamp hours your battery is good for.  If you were using e.g. a 500mAH battery, your were on 1% of the time for 100mA, and your circuit drew 0.25mA when the LED was off, then your average current would be 1.25mA and your battery would last about 400 hours.