The distance at which it is prescribed to shorten one’s prayers and join them
The majority of scholars defined the distance at which a traveller may shorten his prayers; and some of them did not set a limit, rather they left that to be decided by custom (al-‘urf) – what is customarily regarded by the people as travelling is where the concession is granted allowing the traveller to shorten his prayers and break his fast.
According to the majority of scholars, the distance is defined as being approximately eighty kilometers.
It says in Fataawa al-Lajnah al-Daa’imah (8/99): The distance in which the concession of traveling is allowed is that which is customarily regarded as travelling, which is approximately eighty kilometers. Whoever is going to travel this distance or more is allowed to avail himself of the concessions granted to travellers, namely wiping over the socks for three days and nights, and joining and shortening prayers, and not fasting in Ramadaan. If this traveller intends to stay in a place for more than four days, then he is not allowed the concession of travel. If he intends to stay for four days or less, then he is allowed that concession. The traveller who stays in a place and does not know when he will finish what he came to do or how long he will be staying may avail himself of the concessions granted to the traveller, even if that is for a long time. And there is no difference between travel by land or by sea.
From this it is known that you are not allowed to shorten your prayers for the distance mentioned, because it is not equal to the distance of traveling.
And Allaah knows best.