If you have just bought them then yes they will flower from mid spring onwards. If you keep deadheading them they will keep flowering all summer. if you want to put petunias in for summer then yes you will have to remove the violas. i often grow violas in shallow planters and manage to keep them in flower all summer. they make a nice display.

I bought them in August last year - so what you reckon to how long they will last from now?

I have several still doing well 3 yrs on. I trim and feed them regularly.