In an R32 GTR you can expect about the 12 - 13,000km from 595RS, which isnt to bad considering how soft they are, but the problem is that your slowly degrading the tread compound by driving them cold, they are designed to be driven hot, so the more you drive them on the street the less heat cycles your getting into them and in turn you might find that they go a little bit hard after a couple of thousand km's...
That said, if you wear them out you will need to buy more, so go for it, I'll be here when you need another set