If you are serious about earning money online you need to treat it as a business. If you look after your online business it will in turn look after you. There are many different ways you can setup simple revenue streams online, but if you are not taking your business seriously you are more than likely going to fail.
Of course, any legitimate way of making money online is going to require some effort in order to succeed. It is hard to say how much time you will need tp spend; often, just 2 hours per day is enough. However there are certain things that will influence this; your competition, new releases, seasonal updates (thanksgiving etc).
Don't be too caught up in how much time you might need though. It is far more important to make sure that you are doing something and that you keep at it. The main reason people fail to make money is because they don't do this; a lot of people waste their time by reading forums instead of working on their blog or website. Others quit in the first month because they didnt earn anything.
If this concerns you, it is important to remember that there are no "get rich quick" schemes that work. By earning money online you are not likely to see much in the first month since you are only sowing the seeds; however, if you persist and dont give up then you should start to see roots forming from those seeds and they will eventually blossom. This may start with an increased number of visitors at first and not actual money in your bank account but this is a sign you are heading in the right direction.
To summarize, it is more important to make sure that you are taking action and that you persist with your business especially when you are starting out. If you do this, then there is no reason why you cant make a success of your online business.