How long can someone in there early 40s with chronic lymphocytic leukemia live? Another 20 years?

Possible, but varies. The prognosis of chronic lymphocytic leukemia (cll) depends on factors such as the genetics of the cll and the stage among other things. With good prognostic signs some patients with cll can have an average survival of at least 10 years and many will live for 20 or more years. On the other hand the disease can be more aggressive with adverse indicators.
Depends on biology. In about 30-40% patients of cll- people will live in about 10-20 years with cll. In some other cases of cll- depending of the biology of cll , cll can behave more agressively and require treatment earlier. Pso, just like other cancer- prognosis will depend on the biology of the cancer. Discuss in detail with your hematologist/oncologist regarding this topic and prognostic factors to check on you.