Once again this will be dependent upon you and what exactly you are looking for because different profit methods DIFFER DRAMATICALLY in the amount of time that they take to develop and maintain. Obviously you will need to be prepared to put FAR MORE effort in at the beginning of a new project to get it to fly and become profitable than it is to maintain.
However, and once they are established, many of the smaller profit streams I have established require little or no ongoing effort to keep them profitable and adding income to my bank account. Other larger projects (such as this one) require ongoing time in development, testing and constant improvement of the sales conversion process and attracting qualified visitors and prospects.
To give an accurate answer to this question I would have to say that you can invest as little or as much of your time as you wish and HAVE AVAILABLE. As an example let me share with you that my 14 year old daughter has a nice little profit stream which has been established for the past two years and generates her an approximate £250 per month pocket money in return for approximately 30 minutes work every week and she is now in the process of doubling the profit output although the weekly maintenance time will not increase. NOT BAD for one so young!