The short answer to this is roughly twice as deep as they are thick, but as most seeds come in a seed packet they will usually come with instructions on the packet. This is an interesting question though as more often than not self seeded plants that don’t grow in the allocated place for them seem to grow better than their looked after, watered, planted at the right time and nurtured counter parts. At the time of writing this I have just noticed that the broad beans that I planted two weeks ago to over winter are poking through the soil . I also noticed that some that have self seeded from last year (I must have dropped a pod or two when I picked them) are in flower and one actually has bean pods forming. Now these obviously weren’t planted at the correct depth as they weren’t planted at all. So maybe the conclusion to be drawn from this is that it’s not set in stone, although there are rules for these things(or guide lines at least) nature doesn’t understand the rules and grows anyway.