The only reliable way to estimate is to find another relatively similar project and compare it to that. You can say stuff like this new project is roughly similar in scope to Project X but maybe it's about 20% more complicated due to more scope so it will probably take about 20% longer than what Project X took.
The key is to keep data on how long past projects actually took (which not a lot of organizations do). But once you have that real data, you can understand all the unknown unknowns that came up and assume that similar things will come up on the new project.
In my experience that's where story points come in. "This comparable project took this number of story points, therefore <new project> should be similar, resulting in a comparable amount of time." The usage of story points help to adjust for complexity.
The story points are that data point in the past used to indicate the future.
Except if you've already done a very similar project before, the unknowns are now knowns. And more importantly, problems already have developed solutions that can be copied or reused, and not developed. So a very similar project should be an overestimate, and a repeated task should take a fraction of the first time.
The key is to keep data on how long past projects actually took (which not a lot of organizations do). But once you have that real data, you can understand all the unknown unknowns that came up and assume that similar things will come up on the new project.