In the TNG episode The Offspring, we see that Data has built a Soong-type android in the form of Lal, his daughter. It also reveals to us that this was made possible by content discussed at a recent conference that Data attended. My question: can we actually determine how long it took Data to build Lal and, if so, how long did it actually take? (Perhaps working on stardates between episodes?)
Yesterday's Enterprise, the episode before The Offspring, began on stardate 43625.2
During "The Offspring", the stardate when everyone was called to see Data in his lab was 43657.0
ASSUMPTION: Data's cybernetics conference was about half way between those two events, near stardate 43642, so we can estimate about 16 stardate increments between the conference, where Data started working on Lal, to the point where it was ready to unveil.
1000 stardate increments go into a solar year, so 1 day = 2.74 stardate increments. Therefore 16/2.74 = 5.84 days. So Data created Lal in around 6 days.
And on the seventh day he rested.
Apologies for the numerous assumptions. If you take it to the largest possible extreme, that the conference was immediately after "Yesterday's Enterprise", it took up to 12 days to create Lal.
So between 6 and 12 days.