Brake magnests should outlive the linings, but there are some tell-tale
signs to look for:

1) Your trailer pulls to one side when the trailer brakes are applied.
2) You can see wire or copper in the wear face of the magnet
3) The magnet is worn unevenly (the wear is not across the whole face
of the magnet).

#1 could be caused by a number of things, one of which is a defective
magnet.
#2 and #3 are sure indications that the magnets need replacing.  These
can be checked when you repack (or have repacked) your bearings, at
least once a year.

Always replace magnets in pairs.

I have over 20,000 miles on my current brake magnets, including
hundreds of miles in western mountains (i.e. plenty of decents from
6-10,000 feet).  They were fine last spring, but I haven't checked them
yet this year (I normally repack at the start of the season, but it's
been busy this year).

Austin

My linings were wearing out a couple years ago, and the local trailer
specialty shop wanted less than $100 for a PAIR of entire assemblies
(backing plate, retraction springs, star wheels, brake linings, AND
magnets) These were 10" Dexter just like this place has for $80 a pair.

http://abctrailerparts.com/trailerbrakes.html

That's $80 for turnkey, bolt-em in and discard the old ones, total
replacement!

At about $20 for each side for replacement magnets, and $20-24 per side for
new linings, the total replacement idea (four bolts each side and two
magnet wirenuts) was an instant no-brain decision. I've never been more
pleased at how easy it was to choose among the alternatives!

And how cheap!

Here's another source if you don't have one locally:
http://www.lowcosttrailersupplies.com/