If someone had been practicing something like T'ai Chi or QiQong or possible some yoga breathing techniques for years and had developed a sensitivity to energy in their hands already then this might not be the case. I know this firsthand because I taught and practiced T'Chi Chi and QiQong for decades and that was my experience. I could immediately ascertain the energy flow (and see it), but that is not the norm. Most new students are beginners at sensing subtle energies so it will take them longer to get to that place, therefore they need a time frame.
As they progress by doing daily Reiki self-treatments, giving Reiki to others and sending Distant Reiki, they will eventually become more and more aware of the subtle feelings:tingling, pulsing, swirling or a fizzing like feeling) in their hands and fingers, and warmth or heat in their hands.
Eventually they will sense the process of the energy ramping up at first then slowing down and eventually stopping. Or that some areas of the body are just not needing any Reiki at that time (some say that this indicates a blockage in the body but that is not my experience). There is just nothing going on in and under their hands in whatever position they are in at that time, so it's time to move on.
For new practitioners I recommend that they purchase a CD or two that contains bells and chimes that go off at 3 to 5 minute intervals, or songs that last 5 minutes each to help them keep track of time when practicing Reiki. Or they could just watch a clock or count to sixty 3 or 5 times, but it is more more pleasant to use the CD's.
Here's a few I recommend: