Hi there, I always leave mine in and see what happens. Then if they don't re-flower or get leggy I replace them the following autumn. Every year I've got at least two years or more out if mine and often re-flower throughout the year. Just keep deadheading once flowers go over and you might be surprised to see flowers popping up later in the year. I hate to rip stuff out incase there is a chance it might re-flower. I don't mind a leggy plant if it trails over the side with flowers but many would probably say rip them out and add nice new spring bedding. Depends on your budget, time and whether you don't mind waiting to see!?
I collect the spent flower heads just as the seeds are about to burst and then either sow them in seed trays in the spring, or if there are so many I've got fed up with collecting them I leave them on the surface to self seed. The success rate is very high and they all flower abundantly. If plants have got very straggly just cut them back and they may regrow from the base.
I have violas and provided they are cut back after flowering they flower beautifully every year. The current batch are about 4 years old now. Pansies are a different kettle of fish and very rarely look good the following year although quite often they self seed.
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cheers!