Thunderace, using Search is an acquired skill, but having a go gets points. See
here for the results of a search using the keywords "ht tyres", enclosed in quotation marks so the search engine looks for it as a phrase first, rather than ignoring the 'ht' as too short.
From that search I've moved your post to this thread where the OP asks almost the same question as yours.
In terms of life from the factory tyres, probably an average would be around 50,000km - depends very much on how you drive. There are much better quality highway tyres than the factory ones but, like anything, there's a trade off if you want long life. Coopers, for example will give warranty up to 80,000km but the compound is harder. This is common among the US brands while the Japanese brands tend to use a softer compound, so stickier and better in the wet, but not as long life.
Construction type makes a big difference, too. LTs will usually last much longer. Having said that, the same tyres that looked like going 80,000km on my Triton are almost down to the wear bars on my Pajero after 40,000km, despite being LTs. I had a similar problem on my MK where BFGs were shot after about 40,000km, too. I put this down to some tyres being better suited to some cars - weight and suspension type are crucial factors. With that in mind it's best to go on experiences from owners of the same vehicle as yours. What works on a Toyota may not work on a Mitsubishi.