Thanks all but I'm a developer not a professional code reviewer/reader. I prefer to write code and I refuse to sit around reading other peoples code for a year before I get to do my job. It's probably taking this long for them to contribute because after the first week they are over it and don't give a **** about being there anymore. Like Kilgore said, you're pissing money down the drain.
Honestly if it takes so long to get your guys up to speed there's a major floor in your process. I've worked on huge corporate projects involving small and large transactions of millions and millions every day that managed hundreds if not thousands of people and it took me less than a month to start contributing. Call it being pompous if you want, I prefer to look at it as though I have the skills to work things out quickly and employers like that. If it's taking 6+ months like ToxicBunny says to figure out the business logic then maybe you need to go back to the drawing board and rethink the way you have done your systems. If you can't even section off the logic and get people understanding and contributing to that section in a short amount of time your system is a failure. Also you are worried about someone breaking a cash generating system for even 20 mins? I thought you had QA etc to do that? If these things slip past your QA, especially when a dev is new to the environment/system you need to hire new QAs.
I came across an interesting company the other day and one of the architects said that they do not adopt frameworks etc and code everything from scratch. He said it takes them longer to get up and running but they have no hassles now that their projects are running to move and change and do upgrades. Too many companies are so busy being worried about being "cutting edge" that they have 7 projects in MVC1, 3 in MVC2 and 5 in MVC5, one on Joomla, 8 on wordpress, 3 done in PHP raw, 3 using entity framework, 6 in VB6 and 8 in C# as well as that 1 weird one which someone did in VB.Net and no one knows why. Then they also need technical skills because they went and customised some dev express components and now can't upgrade that version on Johnny's computer because it's the only computer that can be used to modify that project. Now the new fad is Angular and Node, Backbone and a few others. Don't get me wrong I'm not against advancement but many times companies make the mistake of adopting a new tech/framework before it's mature and get stuck with a version that they can't simply upgrade and these frameworks wind up running a core component of the business and contribute to long learning times for devs.
Like Kilgore I've also worked in many different industries and not once have I come across a system that takes so long to wrap your head around. Even the large, compilcated systems could be broken down into smaller logical components and worked on from there.