For me, the best time to resign is not when you're bored. Why? Coz in every job, at one point or another, it will really get boring lalo na kung gamay na gamay mo na ang ginagawa mo. I resign only when I am not satisfied anymore with what my company is giving me, whether it's salary, responsibilities, challenges, learning opportunities, etc. It should be a two-way process. Satisfied sila sa nakukuha from you and ganun ka din. If there is a disparity, try to work on it. If it's not workable, resign.
For me, I don't resign when I'm "down". Once I decide that I want to resign, I give my best efforts para when I resign they'll "miss" me. LOL. I also don't resign na walang better opportunity waiting for me. I know I can be replaced by my old company. Ayoko na mukha akong kawawa after ko magresign, no. So I always make sure I have a better job lined up after I quit.