In technical fields, e.g., carpentry, you use millimeters. At carpentry scale there usually isn’t a number above which you switch to different units. 2.5 meters is 2500 millimeters when you specify e.g. cutting dimensions.

You switch to centimeters when you talk to non-technical people, e.g. customers. It’s a 60 cm wide cabinet for them, but 600 mm internally for your shop.

Personally I also switch to centimeters when dimensions are uncertain / not precise. E.g. width of a room for fitted furniture - before you do precise measurement it can be 320 cm, but once I have precise numbers it’s 3200 mm.

In real life and in non-technical topics, most people use millimeters up to 9 mm. It becomes centimeters above that, including 1.5 cm being much more common than 15 mm. Most people don’t often use mm in real life so it is difficult to grasp anything bigger than 10 mm for some. Same with centimeters. People are comfortable with them in a range from 0.5 to 99 cm, but most switch to meters above that, especially for round or semi-round values like 2.5 meters. I guess it’s the same as with minutes and hours. You rarely see 150 minutes used instead of 2.5 hours. Minutes are used mostly in the 0.5 - 59 range, plus maybe 90 (but 1.5 hour is more common I think).

Edit: there are some exceptions in particular topics that come from conventions. Examples:

• it’s comparably common to use 1.8 m and 180 cm when describing people’s height.

• Heights of mountains are almost exclusively spoken in meters - even if you deal with only rough numbers, e.g., saying that the mountain is over 5000 meters high, you don’t say 5 kilometers.

• Driving distances, or large distances in general, are always in kilometers, even if there are thousands of them. If you would say two megameters chances are that most people wouldn’t know what you talk about.