The best way to manage your time is to create a schedule. Decide how much time you want to spend on your website (2 hours a day would be ideal, but it's ultimately up to you), and set the days on which you're going to work on it. Remember, sticking to the schedule is important. You can keep track of it with an app to make it easier (Timetable is a good one), or you can just write it down somewhere and set alarms on your smartphone as reminders.
The most time-consuming part about creating a website is getting started: coding the website (the most intense task), preparing some content beforehand (you can also make it up as you go, but I highly recommend it), setting up social medias and so on. Once you get that done, it will be much easier to manage your time.
Most of your time will be spent producing content. This part really depends on what you're going to talk about, how your articles are going to be structured and how many times a day (or a week) you're going to post them. For instance, running a news website requires less time to write a single article, but you'll probably have to post several daily; on the other hand, a personal blog will have longer articles that could take days to produce, but you won't have to post them every day. I can't say much more about this, for it mostly depends on you, so try to reach a good equilibrium.
Marketing does not take much time. You'll be posting something on social medias, but it won't take long; plus, if you're going to use paid ads, once you set them up you won't have to worry about anything else. I'd say that half a hour a day can be enough.