Solitary wasp larval behavior
It depends on species and situation.
There are a lot of different solitary wasps, the kind you are referring to are parasitic wasps, that lay their eggs in or on a host for the larvae to feed on before they pupate to reach the final stage of their lifetime.
Some larvae that consume a host in groups take as little as twelve days to consume their host and eat their way out.
However, this wasp species can even have their larvae remain in the larval stage through winter and adapt through seasons. it's entirely dependent on what sort of wasp you're asking for what the time span would be for a larva to mature.