Suspension is in most cases not the first response to misbehavour by a user. The first steps are contacting the user and telling them what they are doing wrong, this can be formally via a moderator message or informally in a comment or chat message. For serious incidents, this might be skipped.
The guidelines from SE on suspension lengths are
- 7 days for the first offense
- 30 days for the second offense
- 365 days for the third offense
Suspensions by site moderators are limited to a one-year maximum, though if the user relapses in the problematic behaviour then they can be suspended again for a year. Longer suspensions (often across multiple sites) can be used by SE Community Managers if required; this does not happen frequently but cases do exist.
Those are the predefined values available in the suspension dialog, but moderators can choose any other value if they want to.
Here's a screenshot of the suspension part of the moderator message tool: