"Can reiki really flow in one spot for quite a while? I'm talking about 20 minutes +!"
Yes, it is possible and even longer time, even after you removed your hands.
Sometimes this energy can give you pains that would make you believe that you are sick

) I got that in my arms and the back side of my palms in the beginning, like my bones were hurt, I even went to check if I have rheumatism

)
Sometimes the energy can be so strong that you feel like it is pulling your arms, legs, entire body towards the Earth, this mostly happened while doing the grounding but not only.
I don't think Reiki requires too much concentration, more intention yes, and a lot of relaxation.
It is interesting how suddenly, when I am not even thinking on Reiki or energy, it starts to flow and I wonder then why.
Are you doing the chakra balancing ?