This four year old filly has spent quite a bit of bonding time with her owner in the past few months: going for walks, up and down hills, doing the PNH (Parelli Natural Horsemanship) Seven Games, having grazing time together, etc. She's used to her owner draping herself over her, hugging her neck, being worked from both sides.
The timing of mounting the horse for the first time is determined by when it feels right. There's no set number of hours or training sessions, but only when the owner and horse are ready and comfortable. The owners enjoy the journey and don't worry about "I *have* to get on him next week."
Generally, the first few mountings are bareback in the halter, with a spotter. If the owner is comfortable, the first few short rides in a confined area, will be the same. The horse has previously learned as much as possible on the ground, thru the PNH Seven Games (click here), Dorrance exercises (click here or here), or ground driving (click here) to prepare him for understanding what he needs to know when carrying a rider.
The horse may have already been introduced to the saddle, which this mare has, but we like to introduce the behaviours separately rather than combine the saddle and mounting at the same time. Less things for the horse to worry about.
Linda Tellington-Jones (TTEAM) introduced the benefits of the low head / neck to the horse world a few decades ago. A horse with it's head and neck up, hollows it's back, especially if it's nervous or afraid of confinement. What's nice to notice in this sequence of pictures, is that that the horse never raises her head; she's quite comfortable being able to look to each side to see where her owner is, and then wants to graze. No worry!