"It is a long absence."
Translation:Es una ausencia larga.
Adjectives in Spanish are normally after the noun.
However, when emphasis on the adjective is desired (For your example, it would be a really long absence), then it can come before the noun.
Here is a link to back it up: https://www.spanishdict.com/guide/adjective-placement
The "Temporary vs. Permanent" rule of thumb can be very problematic.
Instead, think of it more like "Character vs Condition".
In this case the "absence" is a characteristic of what "it" is
For an even more thorough explanation, see the link:
just a few lessons back I wrote 'ausencia larga' and duo corrected it as 'larga ausencia'. I went to check the comments and someone wrote that the adjectives that describe quantity goes before the nouns they describe. so I wrote 'larga ausencia' here and duo now corrects it as 'ausencia larga'! Any suggestions?