I clean mine everytime I go to a show! So...that works out to about 5-6 times or more a year. I do a 'deep-clean', which means I take it apart to the last piece and buckle, and scrub everything. Then I polish the brass(or whatever metal it is), and 1-2 times a year, depending on the dryness, I oil it.
Because we live in such a dry climate, I haven't any problems with mold/mildew, so leather, properly oiled and cleaned lasts literally forever. Ref's show harness is about 40+ years old, and the leather is still in excellent shape.
I find it is important to take the harness all apart--this shows you the wear points, the broken stitching/leather, and the bent/breaking buckles and hardware. Its important even with a synthetic--you should check out the whole thing at least once a year.
A dry cloth for dusting will help keep it cleaner for a lot longer, too.
Depending on the amount of time driving and the climate and the horse(ie: sweat amount), you should clean it about 2+ times a year(pref. more). I use a gylcerin saddle soap to clean(with damp sponge), then oil with a saddle/leather oil, then seal the leather against wet and damp with a wipe down of gylcerin. I then do the hardware with a metal polish and soft rag. There are many, many ways to clean and oil harness...what works for me may not work for you.
Yes, you can get them made longer...but I'd be lazy and just get new ones. It really depends on the condition of the current reins--are they 'like new' or worn?