The hyperbole on torsion axle wear is legendary on this site.
The axle life span on a properly sized torsion axle can easily exceed 30 years.
Axle wear should be determined primarily by tire wear.
As a torsion axle wears, it develops progressively more negative camber and slight tow out. These conditions contribute to inside edge tire wear which is the chief indicator for axle replacement.
Of course like any suspension, abuse, overload, or accident can shorten its useable life.
is now in its 11th year and has exceeded 70,000 miles towed. It is normally loaded to about 85% of rated capacity. It is stored properly (most of the time). The axle is in good condition. Predicting its ultimate demise involves an element of speculation, but another decade seems to be a very conservative estimate.
It is now on its third set of tires
with and average service life of 30,000miles. The spare is evenly worn with 40% tread.
A normal torsion axle will take an initial set when new, with additional rotational drop being VERY gradual. Usually tire wear becomes unacceptable sometime after rotational drop at rest exceeds an additional 11 degrees.
Persistent excessive inside tire wear is the primary indication of axle wear.
Even when the time comes to replace an axle the cost of having it professionally done can be as low $500 sometimes with brakes
alone (full kit) would cost at least $200.
Larger trailers would likely be a little more.