While Marc's answer is correct, I think the whole thing can be explained in a little more detail:
To get full access to a tenant (i.e. not just resources associated with the current consenting user), then you need to use the Admin Consent flow. However, unlike other flows in Graph's OAuth, you do not require a refresh_token to maintain access. Instead, you can simply request an access_token at any time once an admin has consented. Each access_token lasts 60 minutes.
Having first used other flows such as the Management API and Graph's Code Grant flow, this one confused me, but it's actually even simpler than those.
To get consent, simply direct the admin to the following URL:
https://login.microsoftonline.com/[Their Tenant ID]/adminconsent?client_id=[Your Client ID]&redirect_uri=[Your Redirect URI]
Your redirect URI will get a response with params:
?admin_consent=True&tenant=[Their Tenant ID]
Once you've received this confirmation, you can request an access_token at any time by sending a POST request with the following form fields:
client_id: [Your Client ID],
client_secret: [Your Client Secret],
scope: 'https://graph.microsoft.com/.default',
grant_type: 'client_credentials'
To the following URI:
https://login.microsoftonline.com/[Their Tenant ID]/oauth2/v2.0/token
The response will contain a new 60 minute access_token and you can simply call it again whenever required.