That's not hundred percent true either. You don't make a map and set a limit wich kind of assets you use and how many of them. Alot of it comes down to creativity. At the moment I would say, that it is easier to make a map settled in France than in Russia for example when you just take a rough look at the available assets. Vehicles and players models are an issue ofcourse, but the map itself is not determined by the assets alone. You can spend alot of time with stuff that is not connected to to any kind of assets at all.
At the end a good map will take a fairly long time, equal whether it's western front or eastern front or what ever, including all different aspects. Proper planning, gameplay, visual style etc. The most important thing is what you make out of the stuff available. Sometimes less is more. Just take a look at Eppeldorf: fairly simple map that doesn't use all Bulge statics and is still a masterpiece.
For a well executed map in all aspects you can easily plan with a half year, if you are fast and know what you are doing. I am not and still in a learning process for example, so for me it takes longer. The question is how easily you are satisfied with your own work, or whether you want to try new/different stuff and push things forward.