You should keep trade confirmations and statements showing reinvested distributions for taxable accounts for as long as you own the shares, and then some. This allows you to establish your basis.
I was given this advice in 1985 before I bought anything outside a retirement account by one of those <gulp> financial planners. Maybe they are not all so bad.
It won't be absolutely necessary to do this going forward for covered shares, but I do it anyway because I have my set of books and the brokerages have theirs.
I am planning to elevate our 2013 income to the top of the 15% bracket this year by selling some shares that have appreciated in order to use the 0% tax on LTCGs in the 15% bracket. At current share prices, it would be about a $48K sale (proceeds) which will produce a $32K LTCG. Without knowing the basis, 1) I would not know how much to sell to get a $32K gain, and 2) I would not know how to handle it at tax time. I have a little unsophisticated spreadsheet in which I enter the anticipated share price on the sale and the desired LTGC, and the sheet tells me how many shares to sell to get that LTGC (using FIFO). I do not yet know exactly how much LTCG I want to generate because of distributions that have not been announced yet, so I will wait until the last week of the year to do this.
Edit to add: If you don't have the confirmations or statements, try contacting the brokerage (EJ). I think different brokerages have different retention policies regarding what they keep, for how long, and what they charge you for providing past statements and confirmations.
For those who view their documents on the brokerage website, it is better to download the documents to your own storage device and maintain them there. There was a thread some time ago in one of these forums started by someone who was relying on a website for documents (CD's at PenFed CU), and through what was essentially a clerical error, he was no longer able to log on to the web site; this caused him some discomfort and inconvenience until it was straightened out.
It really is not difficult to track your documents if you set up a simple filing system using a folder for each account, and have a good naming standard for your saved documents. I use names that contain account number, name, and date in yyyy-mm form so they sort in date order. Start a new set every year. Works for me. I also have three different backups of this stuff on three different media that are stored in two different locations.