Samuel
EASA Part FCL states a minimum of 10 Hours Student solo and states 5 hours of Solo Navigation to include the 300Km(148nm) qualifying solo cross country. It is not any more specific, however, on what constitutes "Navigation".
That is the only stipulation as far as student solo for the PPL goes. It is up to the school HT/CFI to define the actual course and that could be 10 hours solo, of which 9 must be solo Navigation. Usually though, the solo is split roughly 5 hours circuits and 5 hours navigation.
Your Ops Manual should define maximum training periods for students, but again, the PPL Course manual should define how long each flight would be and what it should contain under normal circumstances (and under what conditions these can be changed)
Things are different though, for the LAPL(A).
HTH
DD