The question, as asked, is very nearly unanswerable. Not criticizing, just stating that there's no magic number of lumens that's "enough". There are far too many variables to take into consideration. Other posters have mentioned some of them, particularly location of the landing light on the airframe. The mission-derived variables are huge.
Here's how I looked at it for our aircraft. I operate from an airport which is paved, has good runway edge lighting but otherwise is pretty much as dark as the inside of a cave at night. We also have wildlife which frequently is in the aircraft maneuvering area at night - deer, turkeys, coyotes etc. Our aircraft is a high-wing aircraft (Glasair Sportsman) which originally was provisioned from the factory to accept a pair of MR-16-format halogen lights in the engine cowl. These lights are uniformly understood to be junk - inadequate in light output and beam shape and mechanically unreliable. As a result I have left the dimples for them in the cowl but have not installed these lights.
After assessing "need" versus "want" I bought several inexpensive LED lighting units to assess their performance. Most of them are now installed on tractors and the like since none were really up to the task of keeping our aircraft operations safe. These relatively low-cost test units gave me an idea of what I wanted in terms of total light output and beam pattern.
In the end I went with a Baja Squadron Pro in each wingtip. They are rated at 4300 lumens each and have a tight spot beam. They provide adequate lighting for night ops, including with wet pavement. Adequate, not barely adequate like a typical PAR36, but truly adequate. They still provide more light than any other GA aircraft I've ever flown, and by a huge margin. On short final the entire width of the runway is lit up, plus some of the grass on each side and as I transition from descent to level-off the beams project far enough down the runway that I've been able to safely execute a go-around when I got the glow of deer's eyes and more than a hint if the outline of their bodies a thousand feet or so from the button.
If I had to do it over again I would choose the same lights in the wingtips or perhaps look at the new FlyLEDS option for wingtip installation.
If going with the Baja Squadron Pros again I would opt to install the wider-angle "cornering" lens on one of them to fill in a little bit of the darker area directly ahead of the aircraft. This lens covers two of the four LED emitters, thus the spot beam from half of the lamp would still be available for down-the-runway visibility.
With wingtip lights there's always a convergence point between the two wingtip light beams - in our case that convergence point is likely 150 feet in front of the aircraft. While these lights provide a uniform beam pattern (no huge shadow areas like the typical PAR36 GE4509 incandescent lamps) and decent "spill" outside the central beam, there's still a little lack of surety about what's immediately in front of the aircraft when taxiing on a very dark night.
During the next annual inspection I'll be installing a pair of smaller LED lamp fixtures in the two locations originally provisioned in the Sportsman cowl. These will be used to provide light directly in front of the aircraft to aid in taxiing. They will add about 2000 lumens (1000 each).
Also, the wingtip lights are being wig-wagged using the wig-wag controller from Perihelion. I've had excellent PIREPS and reports from controllers regarding the visibility of our aircraft.
I hope this information is helpful.