I don't, actually. I ran a year long informal experiment on myself, trying to measure my metabolic rate / caloric need per hour, and have a recollection of reading that ingesting a banana before a workout should provide enough glucose for an hour (I don't think it specified for what activity; I found that the banana lasted for the first forty-five minutes of a treadmill 10k run at a pace fluctuating between ~4:30 and ~6:00 minutes per kilometre*) - both probably inform that 15-30 minute figure. Your metabolic speed will vary, and not necessarily within that range.

*By "last", I mean a subjective sense of being energized and having enough fuel. I realise now that may not be a reliable indicator.