If you have the Nomad (Cavalry Warrior) trait and you ride a horse for a full week, it is treated as a warhorse in all respects while you are riding it (in other words, if you are riding a light horse, its statistics change to be those of a light warhorse, and a heavy horse becomes a heavy warhorse: it gets better extra hit dice, better attack bonuses, etc).

It loses this benefit as soon as you stop riding it or are incapacitated. It regains the benefit again as soon as you are active and riding it once more (if you've been separated from a specific mount for a considerable period of time - days or weeks - the GM may rule that you need to "break it in again" and spend another week getting to know it).