Overall, Congress funds public radio to the tune of $100 million a year.
National Public Radio's central organization receives about 2% of its funds from Congress - it could survive a loss of funding as a modest bump in the road. Local public radio stations average a 10% subsidy, but this varies greatly. Urban stations, on average, receive less of a subsidy - a blow but probably survivable if push came to shove. Rural public radio stations, on average, receive more of a subsidy, and many of these stations would be at grave risk of going out of business if they lost the subsidy. Rural public radio stations are far more dependent upon Congressional support than the rest of the public radio system.
A one time endowment of $2 billion to $2.5 billion would be enough to keep public radio funded at current levels in perpetuity.
Alternately, perhaps public radio funding should be parsed into a national programming component, for which an endowment approach that insulates NPR from politics might be appropriate, an urban radio media support grant, and a rural media support grant. Parsed that way, Republicans, the party with stronger rural support, might be more inclined to back funding that is earmarked for their local rural radio stations.
Even if national programming support were lost, private contributions and a little belt tightening might take up that slack. State and local governments in urban areas, where political elites are reliable public radio listeners, might also pull together to save urban public radio stations. But, in rural areas, there is probably neither contributor nor political support sufficient to keep the network alive at current levels. Indeed, it is a fair guess that a decent share of rural listeners to NPR are driving through from one urban area to another and not locals.