This would differ from TOC to TOC, the amounts of tractions you have to learn, the amount of available DI's. It's taking about 15 months at the moment at my depot to get through from start to finish, I'm aware at LOROL that this is less as they have less routes and only one traction, about 9-10 months. But it would be difficult to give an exact time frame as this is all new.
As for pass or fail, yes, people do fail, but if I would give you a golden piece of advice here it is...
1) make sure your DI is challenging you, it's easy for a driving instructor to become complacent and talk about sports or whatever once you get good. However he or she should be challenging you and preparing you for your final. Your DI is effectively a role model driver you inspire to be.
2) if you don't understand anything during rules or traction, ask. There's no such thing as a stupid question, and if you don't understand it, someone else in your group may not, and you don't want to do the mistake of going home and reading up on the wrong section or not reading at all and coming in with completely the wrong information the next day.
3) professional attitude and a try hard approach will assist. You are measured on your personality as well as your productivity
4) enjoy training but remember it's a serious role. So if you are a little bit of a comedian like me, know when to switch this on and off.
5) you need to put in hours of personal time to read up on things to revise for tests etc. It doesn't have to be hours and hours, but it will help.
6) I'm a firm believer of the following quotes.
"Progression defines success"
"Don't wish yourself good luck, but tell yourself never to give up"
I hope this pep talk works. I'm now going to drive round the Hounslow loop.