During first idea, I was a bachelor, and used to invest more time on business development activities, product planning & management, networking events, mentoring sessions, reading books + blogging. I used to spend daily 3-4 hrs of evenings for this. My co-founders were doing coding. I did this for 1 year. Then bcoz of certain concerns, I left this project and started another.
During second idea, I got married, so working in the late evening was not justifiable. I used to work in early morning from 4am-7am, mostly coding and then in the evening, if necessary, meet some potential clients or advisors for business development activities. So daily I used to work for 4-5 hrs. Unfortunately, my other co-founders were married ones, and couldn't cope up with fulltime + mariage + part-time startup pace. So naturally, I was alone at the end, and after certain time, had to close the idea.
Now I again started with the third idea. Now I spend maximum time on learning technical stuff, and coding my application. Most of the times, I am the only one on this idea, I'm spending max time on developing product rather than building business, which I did lot in previous startups. I spend daily 2-3 hrs on weekday, and 5-6 hrs on one weekend.
In general, I try to put 15-20 hrs a week. Which i know are still very less for a startup :( I think having equally entrepreneurial and passionate co-founders, who are ready to slog hard in the part-time is very essential, which was the case in my first idea.