Guys I'm not "certain" by any means, but I'm pretty sure all that "covering the length of your relationship" means is that you cant submit evidence from before you were married. So all evidence "must" cover that period, but you dont "need to show" evidence covering your whole marriage? Thats what I take from it. My wife and I have been together for 9 years(married for 4 and a half). Common sense dictates we have not saved all of that stuff for so long.
Showing you have what you need for your probationary period is enough I'm sure. I might be wrong as always err on the side of caution. But I wouldn't worry about this too much. Think about it if this were really the case, most people would never get their spouse visa.