The distance at which it becomes permissible to shorten prayers
Question

Is it possible to shorten prayers if I know that I will be late coming back? Is the distance for travel 80 km (there and back) or is it only the distance of the outward journey, in order for it to be permissible to shorten and join my prayers?.
Praise be to Allah.The travel in which sharee’ah gives a concession allowing shortening of prayers is that which is customarily regarded as travel, and the distance involved is approximately eighty kilometres. So the one who is going to travel this distance or more on his outward journey may avail himself of the concessions that are granted to travellers, such as wiping over the khuffayn (leather slippers or socks) for three days and nights, joining and shortening prayers, and not fasting in Ramadaan. 
If the traveller reaches the city he has travelled to and intends to stay there for more than four days, then he cannot avail himself of the concession of travelling, but if he intends to stay for four days or less, then he may avail himself of the concession of travelling. If a traveller is staying in a place and he does not know when he will finish his business and he has not set a specific length of time for his stay, he may avail himself of the concessions of travelling even if that goes on for a lengthy period. 
To sum up: It is stipulated that the length of the outward journey should not be less than 80 kilometres in order for it to be permissible for you to shorten your prayers. If you are going to stay more than four days you should offer your prayers in full. 
With regard to joining prayers – Zuhr and ‘Asr, and Maghrib and ‘Isha’ – that is permissible for the traveller, and it is permissible for a non-traveller too if it is too hard for him to offer every prayer on time because of sickness or important work that cannot be delayed, such as a student taking exams or a doctor doing surgery and so on. 
For more information please see the answer to question no.  97844 and 97455. 
And Allaah knows best.