How much should be the bleed area for a banner?

The cutting of banners is more artesanal than lets say some flyers, you do not have a paper cutter board, but a razor. Sometimes is cutted by hand.

So you need to leave more than the average bleed for printing. I would say 1-1.5cm is ok.

You can ask for the borders to be folded and ironed to the back side. You really do not need more that this 1 cm, becouse, although this folding can be several centimiters width, depending on the banner's size, they can simply leave a blank space of another 4 cm arround.