How long do zebra fishes remain transparent?
The upper limit of transparency time of zebrafishes is anywhere from two to three weeks for normal zebrafish for optimal high-resolution imaging. However, there is a modified species of Zebrafish called 'Casper' which is now the preferred zebrafish strain for studies in cancer and other research, because the length of transparency lasts into adulthood.

Sources: 


http://medicalphysicsweb.org/cws/article/research/51394
https://biology.mit.edu/people/lisa_steiner#research_summary
http://www.the-scientist.com/?articles.view/articleNo/34834/title/Models-of-Transparency/