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

We set out with the Prophet ﷺ from Medinah to Makkah, and he would pray two rakahs at a time until we returned to Madinah. I said, “Did you stay in Makkah for any time?” He said, “We stayed there for ten days.”

خَرَجْنَا مَعَ النَّبِيِّ ﷺ مِنَ الْمَدِينَةِ إِلَى مَكَّةَ، فَكَانَ يُصَلِّي رَكْعَتَيْنِ رَكْعَتَيْنِ حَتَّى رَجَعْنَا إِلَى الْمَدِينَةِ. قُلْتُ: أَقَمْتُمْ بِمَكَّةَ شَيْئًا؟ قَالَ: أَقَمْنَا بِهَا عَشْرًا.