I take atenolol 25mg twice a day for my arrhythmia (tachycardia). will i have to take it my whole life ?

Not necessarily: Depends on the cause of arrhythmia. Best to check a cardiologist and find the cause.

?cause: Unless you know the cause of your arrhythmia(such as excess caffeine, overactive thyroid, etc.)and the cause was corrected, you are subjected to the possibilities of sudden appearances of the arrhythmia and accordingly you would want to stay on preventative medication indefinitely. Your cardiologist is best qualified to answer your question, however.