Answer

By Your Horse

Natural Horsemanship Expert Charles Wilson advises:

There are lots of considerations to take into account when deciding how long to leave it before the first ride. The most important thing is to observe the horse – if he doesn’t appear settled, it is unlikely that he will be easy to ride.
As he will be kept at livery, he will have companions. Introduce him to his new friends over a fence for a couple of days before he joins them in a field. If he is stabled atnight, try to have him placed near to his field companions.
Try to find out as much about your new horse as you can, so that you know what is likely to help him settle into his new surroundings, such as a favourite stable toy that you can take with him.
Signs that he is still unsettled include:
● Head up with staring eyes.
● Tail up, moving about a lot.
● Restlessness in the stable, or not eating properly.
● Refusing to connect with another horse or person.
If your horse is showing any of these signs, riding would be risky and it is worth waiting until he relaxes. This could be in no time at all, or a month or so, depending on his character. Spend lots of time with him, just chilling out, or take him for slow walks, grazing in-hand or simply sit near him while reading a book.
All these things will help you form a bond with him. Take it slowly and enjoy your new horse!