If I catch/generate a pet that starts as an adolescent, when will it be a full adult?

pets caught in the wild can be any age from 216 maturity hours through to 648mh (that is, any adolescent age, or adult) so when they'll be an adult will depend on the individual.

Pets made via the generator start life at 216mh, so 18 days.

You can see the exact mh count in the Maturity section of a pet's info if the pet has been genetically tested (either by scanner prior to capture or a test kit after), or if the pet was made via the generator (they come into being autotested just by the manner in which they're made)