I think it is difficult to draw a straight line relationship from a trial lesson to a first solo.
A LOT will happen in the interim, straight and level, turns, climb and descent, stall recovery, approach and landing. Some of these will be knocked off in one lesson others will take some time to sink in.. But, at a time appropriate to how you are progressing (ignore what others do it will vary upon you and a myriad of other factors) you go solo..