The faces on any mechanical seal have to be PRISTINE when put in service. Any foreign objects will cause wear, leading to leakage. Also, the compression has to be correct to keep the faces in contact. Forward gear will attempt to push the shaft forward away from the seal. Lastly, the shaft log and the shaft need to be in alignment to keep the faces flat to each other. Otherwise, you get leakage.
Done correctly, these seals should outlast you. When I was still punching a timeclock, we used this type of seal in lots of different services. Our Mean Time Between Failures was 55 months, running 24/7/365. That's more engine hours than I care to calculate.
Having said all that, I haven't sprung for the new seal myself yet. I'm still using the "It's not broke so why fix it?" mentality for this.