Yearly.  You need to move the trash out to get the water to come through uniformly.  I want 90% or more of my rows to come through with each irrigation.  We irrigate the field 6-10 times per year.  Uniformity need for uniform yields. Make no difference if it is corn or beans or what rotation is used. Yearly. 

You can use a Hawkins hillier.  or We use a Hiniker 5000 or 6000 cultivator with ridging wings. 

 SE/SC Nebraska