Apps like MRTG are configured out-of-the-box to do five minute polling so that they do not overwhelm the targets and the samples are pretty much guaranteed to be accurate (if not as granular as you may want). Plus, the data produced is more manageable.

In general, you should not poll any IOS device's interface stats more often than once every 10 seconds. This might be too aggressive as the device could be working on other higher priority tasks. But, yes, down to 10 seconds, the device should provide accurate statistics.