Estimates, estimates, estimates
How long will that take you?
Now, these are what I like to think of "funding quality" estimates as opposed to "schedule quality" estimates. What's the difference?
Funding estimates only have to be accurate enough to secure enough funds to get enough people to do the project later.
Schedule estimates have to be accurate enough to predict when the project will actually finish.
The latter is much more specific than the former and by the end of the week I'll have had my hands on more than 20 of them since February. Needless to say, I'm quite dizzy and there isn't enough Diet Pepsi Max to go around.
Regardless of which level of accuracy is involved, the same fundamental principles apply, which can be found in my series Making yourself easy to be scheduled, which focuses on schedule quality estimates for the younger techies among us. Namely, experience and repetition are what guide your decisions when making educated guesses on time duration. Find something that you've done in the past that is similar to what you are being asked to estimate and alter appropriately based on it being more or less complex.
Don't know how long it took you before? That's what logging your time is for.
As a high school math teacher once told me, to get good at word problems you have to do a million of them. Same goes with schedule estimates. Do enough different kinds of projects and eventually you'll have enough history to predict just about anything.