The Prophet ﷺ once stayed for nineteen days and prayed shortened prayers. So when we travel led (and stayed) for nineteen days, we used to shorten the prayer but if we traveled (and stayed) for a longer period we used to offer the full prayer.
أَقَامَ النَّبِيُّ ﷺ تِسْعَةَ عَشَرَ يَقْصُرُ فَنَحْنُ إِذَا سَافَرْنَا تِسْعَةَ عَشَرَ قَصَرْنَا وَإِنْ زِدْنَا أَتْمَمْنَا