Here is the basic syllabus
As you can see, randori forms only appear in at the 1st, 2nd and 3rd dan stages. Even though training in them starts at the low kyu grades (as a method of teaching).
A short explanation of the 3 forms of randori:
kakari - no resistance, uke goes over for just about anything
hikitate - not much resistance but uke will only go down if the technique is reasonably effective
randori - bring it on, uke will feint, deceive, move as fast as possible, avoid, string lots of attacks and so on, and can counter (can be tanto or hand attacks)
however, silly as it seems to mention it..even in the relatively stress free kakari-geiko...uke will attack with pace and power.
I have only a few years experience in Shotokan....others more mature than me will be able to more directly answer your question.