The Due's DAC can go up to, and I quote direct from the datasheet:
Up to 350ksps conversion rate
That means you can set up to 350,000 data points on your waveform per second. With two datapoints (giving you a square wave or, more realistically, a waveform with a sharks fin shape) the maximum you could get is (350,000/2) 175kHz.
Given that you stipulate you want a sine wave you're going to need many more than just 2 data points in your waveform. Assuming a (reasonable) minimum of 32 data points (more would give a smoother waveform) you'd be looking at a maximum frequency of about 10kHz. Sacrificing waveform fidelity you could increase that, but the higher you go the less and less it resembles a sine wave.