48 hours.
We had 3 weeks of "manual data collection": pencil, paper and a dozen of people around all the offices of the company with the task to collect serial numbers of every piece of equipment used.
Then we had 3 weeks of data entry, a dozen of people copying all handwritten data to a custom made VB form.

And then there was me, the guy that was in charge of verifying, zipping and sending the data to the client. I spent 48h non stop to go through everything, finding, fixing or delete unusable data.
I had to delete at least 25% of the data because incomplete or completely unusable (serial numbers too short or too long, for example).
48h in the office.

The data was then delivered to the customer. 2 days after, when I finally woke up, everyone was in panic because:
- serial numbers were not matching
- addresses were wrong
- the number of delivered records was smaller than expected

What did I learn from this experience?
When your deadline is tomorrow, and you need 4 weeks to complete your work, ignore the deadline and inform everyone at any level that you are ignoring the deadline. And then resign and find a better job.

Ah, yes, pencils and paper are powerful tools, but rat poison too. You just need to use them in the right place. The only data collection that can be trusted when done with a pencil is the one involving checkboxes.