how many yards should you get out of a new reducer if your pumping through hose with lightweight?

Oh, depends I guess.
A hanging reducer I managed about 25k m3 on.
But I've also worn out one in less than 5k m3 lying down.
But that's with standard rocky norwegian mud. So not so relevant I guess.

But does anyone else get the feeling that the reducers don't wear even? It seems they get oval or something over time.
And that is a good way to get a plug...

I would say it all depends (no, that's not just old people's underwear).  Heat treated, not heat treated, etc.  Some are just made from whatever tubing can be had cheap & those likely can't be heat treated because 'cheap material' doesn't have much (or any) carbon to achieve much in the line of hardness.   A hardened reducer can last up to 2 to 2-1/2 times the life of a non-hardened one in my experience.  I had a boss once that would say they shouldn't be called reducers, they should be called accelerators.  Think about that, wear is exponential by a factor of four.  If you double your yards (or meters) per hour you don't cut life in half, you cut it down to a quarter.  Just like boom systems, there are a whole lot of variables. The longer in length the reduction is, the better, but hanging additional weight on today's booms can be a another problem unless you can reducer the length of your tip hose by enough to meet your manufacturers specs.

BTW I've only seen ovaled reducers caused by sledge hammers, not just wear alone.

Y'all talk'n bout that cone-pipe thingy? The one that goes from bigger to smaller? I reckon there's many a facters in tha e-quay-junn of yer cone pipe wear'n thin? Like what kinda rocks yer runn'n? Round type or angular, is you manufactur'n yer sand or you gett'n tha good stuff right out tha river? Then ocourse you did say lite-wate? That kinds made a holey type rocks by name a poomis er sumthin. No not God fear'n stones I mean tha kind thats plum full a holes & such. I mus say I've yet ta seen a ovulated cone pipe? I seen em triangulated cuz ya whack it with yer leff til it tyers then whack it with yer right for a spell, an shure nuff you goch you a triangulation. I'ma be sure to member this stuff, cuz I like tha sounds a reeducer.  Cone pipe jus dont sound prefeshunall nor proper. Now y'all got me worry'n bout ovulation! I dont think we gots enuff oval type rocks round here to even make 1 load a muud.