Apparently not as long as we think...I just read this from the ZuiderZee article from Holland:
"The stations, completed in February 1930, managed to drain the polder after six months of continuous pumping. "Drained" in this context does not mean the land was wholly dry; extensive pools of shallow water still littered the muddy landscape. To make the soil usable it had to be further drained by a network of drainage canals. Small ditches were dug leading to larger watercourses, which in turn transported their water to the main drainage canals. These canals, dredged when the polder was still filled with water, conducted surplus water to the pumping stations. The resulting dehydration caused the former seabed to sink by over a metre in some places. Once the ground had settled, the smaller ditches were replaced with underground drainage tubes, which would be used for the normal drainage of the polder."
"With the hydrological infrastructure in place, the virgin land was developed to prepare for its later cultivation. The first plant to establish itself, though more so in the later polders than in the Wieringermeer, was reed, sown from the air by plane onto the muddy flats while the polder was still being drained. This sturdy plant helped evaporate the water and bring air into the soil, thereby solidifying its structure and further preventing the emergence of unwanted weeds."
"After the first infrastructure was put in place, the reed was burnt and replaced by rapeseed, turning the newborn polder into a yellow sea of flowers in spring. These crops were succeeded by various grains. In the Wieringermeer the first was rye, but the later polders plant wheat, then barley, and finally oats. This process took years, but once finished allowed planting other crops. At the same time, other infrastructure such as roads and housing were built."
Interesting link. I prefer global warming to ice age since I live in Minnesota.