I would suggest that when a Satoshi is close to parity with a penny (either US, Pound or Euro they are all pretty close), then it is time to add more decimal places.  When 10 Satoshis = 1 penny, it is time to start considered and planning more decimals.

For Order of Magnitude, right now:

0.001 BTC ~= 1 dollar (or pound or Euro)

0.00001 BTC ~= 1 cent.

0.00000001 BTC ~= 1/1000 of a cent.  (A satoshi)

We have 1000 x value multiplier left until satoshi/penny parity.  It took 2 - 3 years for the last 1000 x value multiplier to occur.

We have 100 x value multiplier left until  10 satoshi/penny parity.  It took 1 - 2 years for the last 100 x value multiplier to occur.