When you say ‘Rome Was Not Built in a Day’ you mean that something is going to take
time to complete.
Example of use: “I heard you’re writing a book. When will it be finished?” Answer: “Sometime next year. After all, Rome was not built in a day!”
The origin of the expression ‘Rome was not built in a day’ is an ancient one, attesting to the fact that it takes time to create things that are worthwhile. One of the first examples was published in France sometime around 1190.