Assuming that achieving replacement-level fertility is desirable, is the assumption of 0 years to reach replacement fertility realistic, optimistic, or pessimistic?
Why?
A) Because achieving replacement-level fertility will not drop until people desire smaller families.
B) Because achieving replacement-level fertility is a process influenced by many social factors.
C) Because economic status for large numbers of people does not change overnight.
D) Because cultural preferences do not change overnight.
E) All of the above.
B. is the best answer.
I know it shouldn't be realistic. I don't know whether it is optimistic or pessimistic though.
That depends upon which country and culture you mean.
In some places populations are below replacement fertility. In other places it's above replacement fertility. As countries become more affluent, people gradually have less children. Rereading your question, it's optimistic to assume this will happen in 0 years.
Say India
Indian women still have more children than replacement rate, but it's been decreasing over the last few years.
http://en.wikipedia.org/wiki/Sub-replacement_fertility
https://www.cia.gov/library/publications/the-world-factbook/geos/in.html