We spent 9 nights in Phuket, 4 in Amanpuri and 5 in Trisara. Honestly, we could have spent a few more----I think it's really up to your own preferences. Shuffling around Asia can definitely be a chore at times.

That said, despite all the love for Trisara in this thread, don't rule out Amanpuri - I personally enjoyed it more than Trisara (despite staying in a better room in Trisara) and the resort grounds are truly magical.