When should I stop including talks on my cv?
First of all: note that there is a difference between an academic CV and a resume.


An academic CV typically lists everything you've done related to academia; every talk, every conference paper, every award, every grant, every mentored postdoc, grad, and possibly undergrad.
A resume is a two-page document that summarizes your work/academic experience.


(Terminology may differ, some may refer to the first as a resume also; semantics aside, there is a distinction between the two documents.)

That being said, the answer to your question depends on which document you want to complete. The first should have everything, no matter how old. The second should list your most important, more recent accomplishments, in the interest of space. Regarding the second, the answer to "when should I remove stuff" is simply "whenever you have newer and better things to put in it's place".