This is kind of hard to answer because it is not time that will earn money, it is your high quality and unique articles with good keywords that people are looking for that will earn money. And we don't have the same speed or pace, others can be faster than the other so we cannot say how much time we will spend to earn full time income.
And even we spend long time here in hubpages just answering questions and reading other articles as well as commenting cannot give us money.
It is even hard to say how many hubs you should write to earn because again that will depend if your article is ranking in Googles and having a lot of traffic.
All we know is that top earners are hardworking and been here for years. They accumulated hundreds of hubs others have thousands. So if we like to make a fulltime income we must do what those top earners did and still doing until now. And we are sure they spent a lot of time with all those years they were here in hubpages.