It always takes longer than you think.
As humans, we like to be optimistic about tasks or goals that we have. We almost always think something will be quicker and easier than it is. What this means is, it takes longer than we hoped or expected it to.
In fact, there's a law to describe this, "Hofstadter's law". Which states (in layman's terms) that even though we may know or try to compensate for how long something will take, we will still underestimate it.
This is also the case when it comes to estimating the food you eat or the calories you burn exercising. If you think it was 500 calories, it was probably 600. If you thought it burned 400, it probably burned 300.
So, with this knowledge, how can it be used to our advantage?
Firstly, always overestimate. You'll either be accurate, or you'll give yourself breathing room.
Secondly, understand and accept that things take time. Trust the process and enjoy the journey.