In my experience, Cosmos, once they start flowering, don't really stop - but you do have to deadhead frequently.

I agree.....for me it's watering regularly and dead heading every day. Mine grew into monsters last year - about five feet tall - but they flowered right into November. They are a lovely, delicate but robust annual.

I think it may depend on the variety, Andyfry. I usually plant "Purity" and am almost overwhelmed with thousands of flowers for months on end. Last year I planted 2 different kinds (I can't remember which - "Peppermint Stripe" & "Sweet Sixteen" I think) and was really disappointed with the display: it was pretty but sparse by comparison.

If you are fertilizing your cosmos I would recommend that you do not. This will reduce the foliage and increase the flowering.