Having been around horses for a good many years, I can say that an average breaking time would depend on several factors:
Unbroken: a horse that was born on your ranch. The colt gets to know you, you feed him after weaning. He's like an untrained puppy. He'll follow you around.
Can't be broken for riding until at least 18 mos, with a 2 year old being ideal.
Average time to break well enough for riding, but otherwise untrained: a week. This means, that the horse has taken to a saddle and girth (horses HATE when you cinch the girth) and bridle without freaking out, and then a rider. He has learned accelerate, slow, stop, and directionals.
He has also, over this week of breaking, repeatedly taken the saddle, girth, bridle, and rider, and has performed his core instructions reliably.
For d20, and I'm shooting from the hip here, I'd go with:
Bridle: 15% chance to freak out the first time.
Blanket/Saddle: 35% chance to toss the saddle first time.
Rider: 60-70% chance to buck the rider until off first time.
Now, there are alot of variables here.
First off, even after the week or two of breaking, the horse will not be even CLOSE to 95% reliable, with 95% being the best any horse can ever be.
While riding, being newly broken, stepping on a stick, hearing the SNAP can send that horse into an momentary uncontrollable run.
The horse (most all newly broken horses) develop at least one bad habit like that stick snapping.
He could be a biter. When one grooms the horse, checks/cleans his feet, the attendee can get a good horsebite.
He could be a stomper. He'll try to stomp on the feet of the attendee.
He could be a kicker. This is mainly where the horse simply does NOT like a person behind him.
Most of these habits can be broken over time.
High probability that the first combat or two will bring out more unreliability. He may spook, buck, run, spin, get confused and run into the most severe fighting. Not good for the rider
Also, a creature/monster jumping out of the bushes in front/along side a horse for the first time can easily freak out a horse.
Lastly, jumping. Besides the requisite skills, the horse has a 60% chance of putting on the brakes upon attempting a jump. Unless he has been especially trained to jump, doing so in the wilderness on a newly broken horse can lead to disaster.
I'd look at any jump of ANYTHING over a foot high to be in that 60% range.
To train for true, reliable jumping, takes months. Even a trained jumper can decide "no way" at any time when attempting a jump.
A horse putting on the brakes can be VERY painful. Remember, no seat belts.
For a WILD horse, one that is captured...
At least a week or two MUST pass for some bonding between breaker and horse. This starts with just being accessible to the horse, feeding him, getting close to him and patting him if possible. Eventually, roping, pinning, and saddling/bridling.
This will be a tougher break, in most cases. 2-4 weeks minimum.
you can up to double the % chances for things above.
5% chance that the horse will never be broken. 2% chance that an unbreakable horse CAN be broken by another breaker.
1% chance that you have an absolutely nuts horse, best set free, or used for breeding. No one can even get near him/her.
2% chance that the horse, whether bred, or wild, conforms with the least amount of difficulty.
10% chance the horses habit/quirk never goes away.
You can turn the % chances into skill DC's. Again, I was shooting from the hip, based on personal knowledge, and I'm NO expert, by far. I've never personally broken a horse. I value my skeleton way too much
Look for possible rules in this Deadlands system thats out/coming out.
Also, I'll look into a few of my books. I THINK there are some 2E rules on this, which I'll convert, and return here to post.
Not sure about any existing 3E rules. Hopefully, you now have a few gut guidelines to go by.