When you quit a band, you should treat it as any other professional job. This means let them know in advance, and follow through on any commitments where not following through would leave them in a serious bind.
So if you quit a week or two before a show, be willing to do the one last show if they can’t find a quick replacement. If you’ve got a written contract with a section on leaving, follow the contract. The challenge is that quitting a small band is both quitting a job, and quitting a small social group. If all people are willing to treat you leaving as a professional decision, rather than a social rejection, it is easier to maintain good relations afterwards.
In this case, it sounds like the band is not a particularly professional outfit, if the members haven’t practiced in weeks. This makes quitting professionally a little harder, since there’s no expectation of professional behavior, and if other members are treating you quitting the band as you quitting their group of friends, it is hard to persuade them that really, it’s just the music that you can’t handle, and your decision to quit really is non-negotiable. There is nothing you can do about their reactions, so just continue to make it clear there’s nothing personal in your choice.
You started out well, with telling the leader why you wanted to leave. When he suggested that you stay, and things would change in a month or two and you agreed to stick around, you should have pushed for something definite. So if your biggest grievance was that rehearsals were unproductive (for example), you would give him 2 rehearsals to start improving things, and if he didn’t, you should have told him that things still weren’t working out, and given the date you would quit.
If the leader gets angry, and wants you out NOW instead of the date you suggested, you have no obligation to do that final show with him, even if he comes back begging because he can’t find your replacement in time.
Highlighting the reasons for leaving is a gray area. It is potentially useful to know why someone left, but for a group like a small band, your reasons for leaving are going to come across as personal, even if it really isn’t. If you’ve been unhappy for three months, with no rehearsals in weeks, it’s safe to assume that the other band members know what’s wrong with their group. You can limit your reasons to a simple “This isn’t working out for me anymore. Been nice working with you, good luck in the future.” You’re gone. The chance that your parting words are the catalyst they need to turn their whole act around is very remote, and wouldn’t benefit you in any case. If they specifically ask for your input, give it, but otherwise, keep your opinions to yourself once you’ve left.