I grow onions from seed and use the no-dig method, which reduces weeds. Obviously they blow in from other plots but I hand weed maybe twice a season. If you let the weeds seed you will have them for ages. An old expression is one years seeds seven years weeds. I have no inclination to test the timescale, but its a fact - if you let the weeds go to seed, and they do it remarkably quickly, you will have seeds in the soil and every time you turn the soil over you will bring more to the surface.