I have adrenal tumor. if left untreated, what effect will it have on my long-term health?

Depends: Depends on the size of the tumor and whether or not it's doing anything (producing any hormone). If the tumor is small and it is a non-functioning tumor, you might not have to do anything about it and can live a normal life with no effect on your life expectancy. There are many people who currently has an adrenal tumor and might not even know it.

Possibly: Adrenal tumors are separated into four basic groups: hyperplasia; benign (adenoma), malignant (cortical carcinoma), and metastatic or secondary. If you have a history of a malignancy, it is possible it represents a metastatic tumor. If this is an "incidental" finding, discovered during evaluation for something else, is <4 cm, and looks "classic" for adrenal tissue, then you are probably ok.