How long would a indigo snake be in 17 years?

However long it reached. Snakes do not grow indefinitely, they don't grow x inches in x years, so the 17 year old indigo likely would not be any longer than it was at 8 years old, or 9 years old, or maybe even earlier. One they reach a predetermined growth, all their "growth" becomes microscopic and thus is negligible when applied to quantifiable growth.