1/14/17: I direct seeded Bells mid-October, mid-January, and mid-March. The first round was ready for cutting the first week of May. I don't know exactly when the second round was ready, I want to say maybe a month later.
Because I was only selling to florists at that time, and didn't have a bouquet outlet yet, and I didn't have any orders for them, they sat in my field and grew taller and taller. About the second week of June I began to use them for grocery bouquets and a florist order. By then some of the lower bells had begun to dry, but I just stripped them off and still ended up with good long spikes. Often, with 12-18" worth of bells on a spike, I was stripping off a lot of bells and cutting a lot of stem to make them fit into grocery bouquets. They are the perfect fat, apple-green bouquet filler.
It did rain on them once or twice, and they flopped over. Shaking the rain out the next morning stood them back up again. I didn't use support netting, but if you had a lot you might want to, to avoid the rain-shaking thing.
The third round of seeding never did make decent stems. When it got hot, they struggled, and made little gnarly-looking copies of their larger brethren. When I turned over my winter beds in July or so, they got plowed under.
I did see very nice volunteers in October, leading me to wonder whether a July seeding would make a good round of fall bells.
This last fall ('16) I made just one seeding. This is because I only have 1/2 bed (180 sq ft) to devote to them, and they'll hold in the field for a month while I cut them. If I had a whole bed, I'd do the October and January seedings to spread it out some. They're not exactly once-and-done cuts, some stems still come on after you cut, but they're not endless producers like zinnias.
Postharvest: These guys last forever in the cooler but are VERY geotropic. I store them upright in a bucket that's tall enough to hold the flower spikes, not just the stems.
Also, I really love the way that they smell. The photo is from my first cutting, when the spikes were still on the small side. I wish I had a photo of the plants when they got monstrous. Must learn to take more pictures.