CD4+ cells or helper T cells are produced in Thymus. How long these cells live? For example, RBC live for 3-4 months.
I will respond with a "story-like" approach. You may already know some info but I want to be sure I can help you fully understand!
Unlike RBC, you are born with all the CD4+ (or Th, as I refer to them) you will ever need in your lifetime in the primary organs of the immune system -- they are already specialized and ready to be used, and this has taken place in the thymus. On the other hand, RBC get renewed through erythropoiesis every ~120 days.
In utero, the thymus and bone marrow (primary organs) now supply secondary organs (lymph nodes, the spleen, the MALT) with the various cells they’ve made (B&T). However, only 5% leave the thymus, as 95% are recognized as "anti-self" and are killed. So this would be the first "killing" of Th cells. B cells live in the peripheral layer, while T cells live in the deep lymphoid tissue and the lymphoid dullet.
The dendritic cell will "wander" into the lymph node, carrying the fragment of antigen presentation on the MHC class 2 molecule-Dendritic cell resides between the B cell area and the T cell area. Now, the first specific thing occurs in the immune response, when the T cell receptor attaches to the antigen and tells cell says to make antibodies.
Now -- as of here, we have a lot of Th (and B) cells doing nothing while the antibody is being produced. Some of the Th will be killed, while some will serve as "memory Th cells" so that the next time such an antigen is present in your body, the response will be quicker. At this point, it's hard to distinguish which Th cells are dividing, and which are dying. The average lifespan has been reported to be >17 years. Source: https://www.ncbi.nlm.nih.gov/pubmed/24492253
I hope I have been able to help.