In terms of your first essay at university, I would advise that as soon as you get the essay brief, you start planning for it. Planning is an organic process and it will change as you go through it but you need start researching your subject and planning straight away. So make your plan, go and do some research and then you'll need to revise your plan with what you have found and that's normal that's the reason you have a plan and the reason you review your plan so that you can keep on track and find out where you're at.
Then you write a few drafts, and you will have to change these, and that's all normal as well. And it will take up a lot of time because, as you will be getting used to systems at the university and getting used to where to find the books and you will have to fit all this in with the other demands in your life.
But don't worry about the changes that you will make, and don't worry about the time it will take, because this is how the essays are written. So, I would advise though that you spend plenty of time, sorry, allow plenty of time for them.