I concur, 15' should be plenty.
To wire up each sub within the enclosure, shouldn't take but 2', then you need just over 3' to get from on terminal cup on the enclosure to the other, which leaves you roughly 8' to get to your amplifier. However, if you are planning to do two runs of wire (one to each sub, then you'll need to double the length of the run from the amp to the first terminal cup added to the total of needed wire.
Nevertheless, unless you have the amp located 4' or more from the enclosure, then the 15' should be enough. "IF" you have any doubts that it won't be, order more from the start. I'm sure you'll find a use for any extra wire over time.
Also, while you never mentioned, nor was it ever asked, are you planning to wire the subs for a 1 ohm final load and do one run of wire? How much power you running? Will the wiring be ran concealed?