percentage of customers reply

hey, i need help with my work report, can't find right formula for it.

so here is the thing, i work in a blog where customers ask questions and i have to answer it during exact time, as below

from 7:00am till 3:00pm i should answer within 1 hour
from 3:00pm till 10:00pm i should answer within 3 hours
from 10:00am till 7:00am i should answer within 10 hours

so for sure I'm not replying all customers at same time, i need to calculate the percentage of the time to know my KPI.

i'll put example if it needed to help with finding the formula:

3/14/11 8:37 - 3/14/11 8:57 0 Days, 0 Hrs, 20 Min,
0 Sec

3/14/2011 10:03 - 3/14/2011 10:22 0 Days, 0 Hrs, 19 Min,
0 Sec

3/14/11 8:37 - 3/15/11 10:57 1 Days, 2 Hrs, 20 Min,
0 Sec (so this one i answered the customer late)

help !!!!