Once they start flowering in late August/early September, they'll just go on and on until the first frosts.
Perfect in patio pots, they also make excellent subjects for edging a pathway. Leave them in situ to die back over winter, cut out the died-back stems when new growth appears in the spring and the flowering cycle starts all over again. A rich reward indeed for so little effort!
Available from www.marshalls-seeds.co.uk