Both of our services, 9am & 11am are about the same length, 1 1/2 hrs. First service is a little more time restrained due to having to get people out for the arriving 2nd service folks. We normally use all the time plus a little bit extra depending on how the service ends. We don't have altar service every Sunday, but when we do, we don't worry about the time.
I don't know why we didn't go to two services earlier. Our attendance has tripled over the past 3 yrs. God is good!!