In general, though, when you rhodium plate (or any plate), you are
actually taking metal out of solution and putting it on your work.
We all wish that the metal in solution was eternal, but it's not.
There's no free ride, especially with rhodium. Now a friend who
plates a lot keeps his old solutions that are clean and lets them
evaporate till they turn orangey-brown, and uses them for
replenisher. Just one way...
As John wrote, his friend lets his used solutions evaporate until
they turn orangey-brown. As a chemist, that’s really the only way I
can think of (without huge pieces of extortionately expensive
analytical chemistry type equipment to measure things accurately) to
make use of previously used plating solutions. As he says, you are
taking metal atoms out of solution and putting it one your work and
so there is less metal in the solution once it has been used and so
the solution is a much less concentrated metal solution. But, as his
friend does, if you let some of the solvent (the liquid the metal is
dissolved in) evaporate, until it is roughly the same colour as it
is intended, then you will achieve a roughly correct concentration of
metal solution that you can use to add to the plating bath. You just
have less volume, but it’ll make your plating solution go further
than it would otherwise.