This thread motivated me to look at my 40 year old logbook. I originally started my PPL with about 70 hours of gliding and soloed the Tiger Moth after 2 hours 45 minutes of circuits (plus just over an initial hour covering the rest).
I gave up flying completely for 23 years (kids do that to you) with about 100 hrs total power flying.
When I took it up again I had to do the complete syllabus again and, this time, I took 2 hours 40 minutes of circuits in a C152, almost exactly the same as before. This was at a much bigger airport and circuits took somewhat longer so a lot fewer landings. This probably reflects the easier handling of the tricycle C152.
So, I would guess that 30 - 50 landings are a reasonable number to train for solo depending on the type of aircraft and runway. This would assume that basic handling is reasonably competent by then.