I just sow about 20 in Feb and then again another 20 once they are about 3 inches tall, and keep doing that until late April. After then, they won't produce much come the summer.
I plant 20 out, with about 3-4 inches between them, in a block and put 4 canes in, at each corner. Then I put twine around the canes, so boxing them in, starting at the bottom in a spiral with about a foot in between each spiral bit. I tie it in to each cane as I go round. So they effectively grow in a tube.
Or I plant them in a line, put one cane at each end and again, put twine between the canes with about a foot between each line of twine. The height of canes and twine depends on the height of the final pea/mange tout plant. My Goldensweet Mangetout are about 3-4 ft tall whereas the Carouby De Maussaune can get up to 7ft tall so they get different cane heights. I'll also add others in as the season progresses if they go mad, to hold them up. And more twine if necessary.
I don't put it in the same bed as I split all my crops up around the place. So one batch will go in one space, the next in another and the next in another. This time of year there is quite alot of space waiting for things to go in.
Sowing til harvest, depends on variety etc. They all really get going once it stops frosting so the earlier ones will not be the same amount of early as the later ones, as the later ones do start to catch up. I'd say about 12 weeks from the early ones, to maybe 8 with the later ones.