Estimating the amount of time it takes to write documentation is tricky as it relies on many differing, subtle, factors and, for many people working outside of a highly regimented and heavily project managed team, it tends to boil down to a mixture of guesswork and experience. However, it’s not impossible to come up with a more reasoned estimate as long as you don’t mind doing a little planning.
McLean, Gordon. One Man Writes (2009). Careers>Documentation>Technical Writing>Estimating