Riding daily is almost a requirement. At this age, consistnency really helps.
How long will it take to get him solid, stable and safe? A lot of that depends on the horse. They learn at different rates, just like people. And what I consider solid might be different than your definition. What I look for in a solid trail horse is one that is willing, calm, adaptable, and responsive. For a really solid trail horse, I want one that I can ride one-handed.
To make a trail horse, I start out going down the road. One of the neighbors has geese that like to run at the horses and honk. We have a lot of pheasants around, too, so they get pretty used to birds flying up from right in front of them. When they get to the point that I trust them a little bit, I load them up and take them to my friend's farm. He has some pastures that are a lot rougher than the land around my place. I take them up and down hills, through all kinds of water, and along narrow trails.
With daily riding like that, it took me anywhere from six months to a year to make one solid (apart from the curb bit).
Learning never stops