A maximum of two weeks. Boy, I'm a negative person today, aren't I. I hope you get some other input, also, to compare. You're quite a bit cooler than we are, so they might last longer for you.
They might have some early pansies in the stores, too, that would do well for your project. It's one of the first things we have here.
To be on the safe side I would do quite a few plantings of the crocuses and bring them in at different times.