For the 744 (from our FCOM):
Two IRUs in conjunction with one FMC and two FMS-CDUs meet the
requirements as the sole means of navigation for flights up to 18 hours duration.
There are no time limits in OpSpec B036 or B039, as long as GPS or DME/DME updating is operational upon entering airspace requiring LRNS (Long Range navigation System), and dispatch requirements were met.
HOWEVER, such [lack of] limitations are operator-specific, per OpSpec B039:
c. Minimum Navigation Performance Capability Required. The certificate holder shall not conduct any operation in NAT/MNPS Airspace unless the certificate holder has satisfactorily demonstrated that the navigation equipment is installed and operational on any airplane used in NAT/MNPS operations. The procedures for use of this equipment must meet the following NAT/MNPS requirements on a continuing basis:
(1) The standard deviation (one sigma) of the lateral tracking error is less than 6.3 nautical miles (NM).
(2) The proportion of the total flight time in NAT/MNPS Airspace spent by aircraft 30 NM or more off the exact centerline of the assigned track is less than 5.3 x 10E-4 (less than one hour in 1,887 flight hours).
(3) The proportion of the total flight time in NAT/MNPS Airspace spent by aircraft between 50 and 70 NM offtrack is less than 1.3 x 10E-4 (less than one hour in 7,693 flight hours).
(4) Suitable displays must be available at each pilot's station to permit continuous monitoring of the long-range navigation systems cross-track and along-track information.
The 744 DDG simply refers to the FCOM Limitations section for Dispatch and entry requirements. Again, no specific time limitations for degraded systems AFTER entry.