Is it good to have milestones as a countdown for internal processes? For example, let's say we don't have sla with a customer, but we want to have internally a rule to support that we will support him within 2 hours. Are milestones the way to go or is it not a best practice?
For your scenario, the Milestone will work,
The above link will guide you to set up a milestone.
If more than 24 hours remain on a milestone, the countdown displays in days (for example, 1d). When fewer than 24 hours remain, the countdown format switches to hours/minutes/seconds.
For this, an idea is also raised,
Business Hours Work in Entitlement Management will work for you.
I hope it helps.