To get 400 Microsoft points you need 500 Bing points. They change up the amount of points you can earn a day by searching so the time needed to get to 500 Bing points vary. Currently it's 15+1 bonus search a day which takes about a month to hit 500 bing points.
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At one point they gave me an opportunity to get 33 points a day which was 400 Microsoft points every two weeks.