They will flower again next spring. If they're in pots, plant them out in a corner somewhere and you'll be able to use them again. If in the soil, just leave them. They might seed themselves.

They flower for about a month or so, I'd say, in the spring. You could cut or pull the old flowers off, but it isn't necessary.

its bite late now but usually when the first bloom is finishing i dead head and atleast half of mine flower again. but for now like spritz said just let them die back and gain a bigger display next year - sometimes they flower again in autumn i think.