imho it's more economical to shave. 5 mins on the face twice a week. 5 mins 1mm trim on the head every 4 weeks. think of the time & money saved.
How fast do they grow and how often do you change the blades
[my father in law refuses to change them till its worn, which is totally unhygienic? No?]
once the shave doesn't feel smooth enough - typically 3 months :-)
that's cos you've got the zz top look going for you. in the local heat & humidity, a beard just means hygiene issues.