Estimating how long a certain task will take can be challenging. Here's a few principles to keep in mind when you try and estimate routine tasks or complex work you've never done before.
Leverage Parkinson's Law
When you are trying to "estimate" how long a complex task you've never done before might take you are better off applying Parkinson's law and constraining the work rather than trying to estimate it.
Parkinson's law states that "work expands so as to fill the time available for its completion". Basically, you'll finish the task in however much time you give yourself. To leverage Parkinson's law simply pick an ambitious but not absurd timeframe to finish the task within.
There's a powerful side effect that comes into play when you pick an ambitious time frame. You are forced to find ways to get the job done in that time frame. The simplest manifestation of this that you'll trim the scope of the work to just what's essential.
Account for underestimation
In the case of simpler more routine work, it's almost certain that you will underestimate how long a task will take when you are new to time estimation. Give yourself breathing room and add 20% to how long you think it will take.
Set a minimum
Once you start estimating and tracking time, a common observation is that all those little tasks that we don't consider consequential actually occupy significant chunks of time. Instead of letting them pile up in your daily list, accept that they'll actually take some amount of time and always estimate at least a few minutes for them.
Remember that you have limited capacity each day for even the simple and routine tasks. Even in an eight hour work day you can only do 32 tasks that will take 15 minutes.
Be wrong
And finally, the best technique for setting good time estimates is to just accept that you'll be wrong in the short run and that it will take a couple weeks before you are more accurate.
Break it down
If you can't figure out how long something will take, break it down into subtasks and assign an estimate to each subtask.
Create a milestone
If you've got a longer term project that's going to need to be completed over several days and you aren't even sure what will need to be done until you've started exploring you can start by just adding a single subtask that represents your daily milestone and how much time you'd like to spend on it.
