We have had five of these in our workshop for 14 months, running at full speed 24 hours a day, with no problems.
As they will be used to keep your watch charged, they only run for 8 seconds every 8 minutes (1/60) at 3/4 speed, so in theory they should last well over 100 Years.
Unfortunately, we are unable to give a 100-year guarantee as we won't be here. But we will give a full two-year guarantee.