How long is it normal for two brothers to have a sibling rivalry?

Unknown. Unfortunately, there is no definitive answer to your inquiry. Potentially for the rest of their lives is the reality of the nature of 'brotherhood'. There may be periods of competition, rivalry, arguing, etc...As long as the love between them is emphasized; and they should be taught that respect of one another is of paramount importance; then "brothers will be brothers" as they say.
Ask Cain. It can last a life time, although usually after late early adulthood things quiet down a bit.