As mentioned, it really depends on what you mean by everything. Can you see every parade, show, character, ride etc during that time? Probably not. Especially not if you remember that there is DTD, BB, TL, mini golf etc. Can you do most of the major attractions and highlights? Definitely. As a PP mentioned, look into the Unofficial Guide, and possibly a subscription to Touring Plans. Simply by going in with a plan, you can help to maximize your day and minimize your frustration. Doing the research in advance can help you to know what rides you absolutely want to do, and which ones you can/should skip. You may happen to not be a big fan of roller coasters, and could then easily rule out SM, BTMRR, RnRC. Or you may be a Safari guide in real life, and then could skip Kilimanjaro Safaris.