There is very little chance that what you are working on has never been done before. Most likely, you are writing simple code to do simple things. These things can be broken down into simple tasks, and these simple tasks can be estimated.
Unless you are working on ChatGPT-6, i assure you, you can definitely estimate how long things will take. your estimates will not be perfect, but they will be good approximations for guiding development.
Even OpenAI by now probably is pretty good at estimating how long the next ChatGPT version will take to develop, since they've done it 4-5 times already.
Alway always distrust someone who says "we are doing things that have never been done before". At the very bleeding edge (OpenAI, SpaceX), you are iterating on something that has been done many times before, but with a small innovation.