Since you qualified it slightly I voted "1 month"
I think the first month(s) after a pilot is soloed are some of the most dangerous for the plane. Many newly soloed pilots are brimming with confidence and wanting to wring out the trainer. The only way to learn new things is to try them. Trying them with an instructor nearby is best but I think a lot of people hesitate to "bother" the instructors after they solo. I think most instructors would actually be very happy to continue helping a pilot who wants to improve. One of our club members is a Masters level pattern flyer and also is a really good instructor. He loves to help pilots learn aerobatics. He will help through a trainer box or just by talking through the maneuvers.