The allium heads will stay pretty much as long as you want them to stay. They will go well with your phlox and give a lot textural interest to your garden. There are all different types of alliums at all different heights that are available.
The only alliums that I have, that do not produce a round head are the Flavum and the Pulchellum. I love these as they remind me of fireworks and their bloom time is later than Globemaster or Jade Eyes.
Alliums are the one of the bulbs that I continue to add each fall. The flowers are so different and beautiful when they are in flower and the interest they provide to the garden after they are finished flowering is well worth it. They are also a very easy plant -
