The normal breakeven is around 10 days. But it will really come down to your specific travel plans. While a 4-5 day trip once a year won't make it worth while, planning your trips so that they fall within a 365 period of AP validity might make it worth while. IE travel in late Aug in 2017, early Aug in 2018.
A 4 day base ticket currently goes for $350 at the gate. A Platinum AP goes for $779 (tax not included in either). So two 4 day tickets would cost you $700, and you could very well make up that $79 difference in things like Memory Maker, merchandise discounts, etc. Keep in mind that all those extras are subject to change, so they may very well not exist 6 mo to a 1year down the road. Look at the prices for the tickets you would conceivable need to buy, and if you can make trips overlap, then it might be worth it to buy an AP. And if you have a family, you may want to buy just one AP to get anything such as Memory Maker and buy straight tickets for the rest of the party, it all depends on the numbers.