How do you schedule the time you will need to do a task at your work?

I’m working as software engineer and all the time I am beening asked how much time it will take me to do something like: writing a new code, fixing a bug or write a document and more.

But, I never know really how much time it will take me. there are things I can’t predict that happens at the process.

What is your answer when you need to give a specific time?
How you estimate the time for a task?