First time Brewing Cider

On Saturday I began brewing my first cider. I purchased cider from a local stand and it didn't say if it had or didn't have sodium benzoate or potassium sorbate so I used it anyway. I had some activity pretty quickly but the bubbles werent really the prominent. It is now Wednesday and the bubbles are very small on top and there is no activity in the cider. I know the gap will range between as to how much gas is being produced but now there is a 15 to 16 second gap between air bubbles passing through the airlock. After day 2 it was at 4 to 5 seconds pretty consistently. Room is 65 degrees or so at all times.

Is this brew done for or should I let it keep going?

Let it ride and get a hydrometer. Bubbles in the airlock can indicate that fermentation is happening, but the only way to know that it is done is to use the hydrometer. Finished cider gets down pretty low, so your yeast probably have quite a bit of munching left to do before they are done. Even if you didn't measure original gravity, a hydrometer will still be handy. Your final gravity will probably be very close to 1.000, so when you see a steady reading in that range, you will know that your cider is done.