Always double your level of effort estimates to any management over what it's really going to take. Always.

And sometimes, add a zero.

And even more importantly, split it up in tiny pieces. "I really can't tell you when the new statistics page with all the company metrics will be ready. But by lunch you'll have a login button to click on, and at the end of the day it will show a single graph which represents the negative relation between people asking me to write estimation reports and my coding productivity. Then we'll take it from there"