Fava plants typically start to flower about six weeks after sowing. At this point, you can start to trim the tops for their tender greens.
To do this, pinch or snip the first few inches (or the first two to three sets of leaves) from the top, flowers and all, just above a leaf node (a junction on the stem where a young shoot is starting to emerge).
I only pinch off the leaves and flowers once every couple of weeks, as I do want the plant to focus on producing beans instead of generating foliage.
After your plant has produced a full crop of mature beans, you can cut the whole plant down to the ground, leaving about 6 inches standing.
Always ensure you prune above a leaf node where you see young shoots starting to sprout. This vigorous haircut often encourages the plant to grow again and produce a second crop of beans.