You can make some good guesses about levels of unpredictability based on previous experience though.
For example:
If the job involves dealing with anyone from Samsung or LG, it will take 2-3 times as long as you think.
If the job involves adding in a commercial third-party component which has a website about it, and that website doesn't clearly explain what the benefit of the thing actually is, the job will take 10-20 times as long as it should.
If the job involves a new feature Amazon or Google have just launched on one of their devices, and you are being CC'd on email chains involving representatives of Amazon or Google who are claiming implementation is very easy, the job will take a minimum of 6 months and may never be completed because the feature will be dropped or replaced with V2 before V1 ever actually works.
For example:
If the job involves dealing with anyone from Samsung or LG, it will take 2-3 times as long as you think.
If the job involves adding in a commercial third-party component which has a website about it, and that website doesn't clearly explain what the benefit of the thing actually is, the job will take 10-20 times as long as it should.
If the job involves a new feature Amazon or Google have just launched on one of their devices, and you are being CC'd on email chains involving representatives of Amazon or Google who are claiming implementation is very easy, the job will take a minimum of 6 months and may never be completed because the feature will be dropped or replaced with V2 before V1 ever actually works.