My kids are in the pool once a week too, and I wash their hair right afterwards. For me them being in the pool is the main reason *why* I wash their hair! Chlorine is so hard on hair! (and skin...) During breaks from swimming lessons we do often go longer than a week between hair washes. Dd gets her hair washed more often than ds because the conditioner really makes it easier to get the tangles out (ds has short hair).
Besides that we have weeks with no bath (besides post-pool shower), and weeks with 3-4 baths. It depends on a variety of factors - dirtiness being one, and also that they sometimes "need" a bath to help mellow out and calm down before bed time.
I would wash hair right after pool and my dd needs it once more mid-week now or you can smell that she needs it washed. She also needs to wash under her arms too now, so this is a recent change. Before, I'd be OK with once a week.