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  

I heard Anas saying, "We traveled with the Prophet ﷺ from Medina to Mecca and offered two rakʿat (for every prayer) till we returned to Medina." I said, "Did you stay for a while in Mecca?" He replied, "We stayed in Mecca for ten days."  

سَمِعْتُ أَنَسًا يَقُولُ خَرَجْنَا مَعَ النَّبِيِّ ﷺ مِنَ الْمَدِينَةِ إِلَى مَكَّةَ فَكَانَ يُصَلِّي رَكْعَتَيْنِ رَكْعَتَيْنِ حَتَّى رَجَعْنَا إِلَى الْمَدِينَةِ قُلْتُ أَقَمْتُمْ بِمَكَّةَ شَيْئًا قَالَ أَقَمْنَا بِهَا عَشْرًا