At what klm stage should lifters be replace.

Thats is absolutely impossible to answer.
I have seen stock evo lifters, for example, go for close to 400,000 kms(396,000 in fact), and though, of course, there was wear, they were showing no signs of being about to fail.
The engine had experienced an easy touring life, and had never been opened. Stock HD crankpin, and it was not chopped out.
Depending on the motor series, cyl head mods, cam, service life, and even riding style,etc, the dynamic of lifter life is going to change dramatically.
I would say this, however.
If we take an EVO, again for example, then we know that HD states that the evo top end is designed to perform for at least 80,000 kms without major service. Should you find yourself freshening up the top end(at whatever stage), then even if you had no real cause to suspect lifter issues, it would not be too extreme to include a cam chest inspection along with other work being done.
Do I think it makes sense to lay down a blanket mileage at which people should take their perfectly sound engines apart, and chuck out parts that have absolutely nothing wrong with them?
No, of course not, as that would be ridiculous.

Kiwidave, thanks for the advise, will check the ittle suckers out next time round. have a good one.  

DynaRider, many thanks for the reply, will check the little suckers out next time round.

A bit more on lifters mine has reached 91,000 ks am going to Isa in August,to my knowledge a bit of top end noise but am running V Twin oil so expect that, am told you will get about 100,000ks out of a set, or do i go by the old saying if it aint broke don't touch it.Buttttt it's a long way between drinks if it let's go up here.Hmmmmmmmmm. 03 Ultra sorry.charlie