This Wikipedia page ([Decimal Degrees](http://en.wikipedia.org/wiki/Decimal_degrees)) has a table on "Accuracy versus decimal places". Also the accuracy of your coordinates depends on the instrument used to collect the coordinates - A-GPS used in cell phones, DGPS etc.

    decimal places   degrees      distance
           0         1            111   km
           1         0.1          11.1  km
           2         0.01         1.11  km
           3         0.001        111   m
           4         0.0001       11.1  m
           5         0.00001      1.11  m
           6         0.000001     0.111 m
           7         0.0000001    1.11  cm
           8         0.00000001   1.11  mm