NASA appears to have settled on a Near Rectilinear L2 Halo Orbit (NRHO) for its future Deep Space Habitat (DSH) which its now calling, the Lunar Gateway. NRHOs are a subset of of L1 or L2 halo orbits. NRHO's have large amplitudes over either the north or south lunar poles with shorter periods that pass closely to the opposite pole. Station keeping at an NRHO would require a delta-v (change in velocity) of only 5 meters per second (5 m/s) per year. With an impulsive departure from LEO (Low Earth Orbit) at about 3.124 kilometers per second (3.124 km/s). So a crewed spacecraft from Low Earth Orbit would reach an L2 NRHO in approximately 5.33 days. Orbital capture would require an additional delta-v of 0.829 km/s.
An EML1 location for the Lunar Gateway would only require a delta-v of 3.77 km/s and four days of travel time. But 2 days of travel time would be required for a journey from EML1 to Low Lunar Orbit (LLO). An NRHO location, however, would only require 12 hours of travel time to LLO. So the surface of the Moon could be accessed from a NRHO located Deep Space Hab in just 12 hours.
Possible Cis-Lunar Locations for a DSH (Deep Space Habitat)
EML1 (Earth-Moon Lagrange Point One):
Travel time to and from LEO: ~4 days (3.77 km/s)
Station keeping: < 10 m/s per year
Travel time to and from LLO: ~ 2 days (0.750 km/s)
EML2 (Earth Moon Lagrange Point Two):
Travel time to and from LEO:~ 8 days from LEO (3.43 km/s)
Station keeping < 10 m/s per year
Travel time to and from LLO:~ 3 days to LLO (0.8 km/s)
DRO (Distant Retrograde Orbit):
Travel time to and from LEO: ~ 6 days
Station keeping: 0 m/s per year
Travel time to and from LLO: ~ 4 days (0.83 km/s)
NRHO: (Near Rectilinear Halo Orbit):
Travel time to and from LEO:~5 days from LEO (3.95 km/s)
Station keeping: 5 m/s per year
Travel time to and from LLO:~ 12 hours to LLO (0.730 km/s)
Significantly shorter flight times from LEO to NRHO could be achieved with higher delta-v levels that could easily be achieved by future reusable LOX/LH2 fueled spacecraft such as the ULA's XEUS and Lockheed Martin's MADV which could be used for round trip journeys to the lunar surface from a NRHO and for transporting crews between LEO and NRHO.
Links and References
Options for Staging Orbits in Cis-Lunar Space