Whatever you find in the books, in the U.S. at least, it's going to depend on other factors such as the type radar available, and staffing etc. For example, my facility has parallels separated by more than 6000', and we have the hardware for simultaneous approaches, but do not have the staffing nor procedures in place for two final positions and a monitor. We just don't suffer that many delays with IFR weather, as much of our traffic is VFR. If we ever do start getting seriously backlogged in IFR, we'll likely get to work on that....