Estimating how long a task takes, for instance in programming, is always difficult.
Here is a bookmarklet that you can use. Create a bookmark with this as the url
The algorithm used is this:
t1 = how long it takes if everything goes as planned.
t2 = how long it takes if there are some problems along the way
t3 = how long it takes if everything goes wrong (this is the time that you are 99% sure it takes to finish it even if everything goes wrong)
(t1+(4*t2) + t3) / 6