The length of a shows run isn't as important as the quality of that run. Sometimes shows go on too long and quality can suffer as a result. The problem with ENT is that despite a contining decline in audiance figures throught the previous Trek shows (for a vareity of reasons). They simply more or less kept doing the same stories they had been doing as if the result would be different (i.e viewers not turning off). By the time they did actually do something different S3 and actually really commit to a prequel show S4 it was too late.