Before sending a student solo, you need to see three consecutive safe circuits and landings, the probability then is that the fourth will be much the same.
The student has to demonstrate that the aircraft is under control at all times and that they can cope with simple emergencies in the circuit.
The time taken to go solo will depend on a number of factors, ability, opportunity, age, instructional standards and frequency of lessons. In isolation it means very little.
The judgement required by the instructor takes time to acquire and for that reason the AFI or FI(R) may not send students on their first solo. For an instructor, deceiding when to send their first student solo is a challenge, as experience grows it is quite easy to know when, a feeling in the water!