Well, only if the existing one fails. Its not like a car where you have to have the tyres changed after so many miles. You just keep going and as long as the bypass is still functioning then there is no need to do anything. In many cases, if a bypass vessel forms a blockage, a Stent can overcome the problem.
Some bypasses last weeks, some months, some 10 years and some even longer. There
is no set expiry date.