With the older INS (such as Delco Carousel C-IVA) - 1980s
Using triple mix and DME update...
On typical NAT track between YQX and SNN, was nearly 4 hrs without DME update.
About the same time between NHA (Noronha) and SAL (Cape Verde) DMEs
From US West Coast to HNL was somewhat closer to 5 hrs.
xxx
It took the GPS update to make these old INS compliant with RNP-5
Happy contrails