When do you start the next story?

When do you start your next story?  Part of me is itching to start the next one, yet trying to find an agent for the first one.  I feel like I’m abandoning the first one, like the sack of junk I left at the Goodwill last week.  My heart wants to stay with those characters, while the brain is yelling at me to stop ignoring new ideas.  How long did any of you wait before you started? Also, I am in week four of querying, with 7 rejections, one partial (still waiting), and the rest I haven’t heard.  Okay, so that’s two questions: how long did you wait before starting the next story, and how long did it take get an agent?  Thanks, and happy Sunday!