Linksys has to my knowledge never said officially (unlike Ubiquiti which has it in the data sheets) the rough number tends to be around -86dbm however again never officially defined.
If you are plotting new sites very much recommend looking at the Ubiquiti gear instead, you can end up with a ~20dbm link budget increase (depending on local factors) just by switching to a Ubiquiti device, in addition the receivers just seem to perform better overall even in locations where the noise floor negates that sensitivity advantage.