Q: What are the exact conditions to combine and shorten your prayers when traveling? (Distance, days until you’re not a traveler anymore, etc.)
A: It is recommended to shorten a four-rakʿah prayer for anyone who intends permissible travel (including tourism and sightseeing) to a specific destination which reaches 16 farsakhs (i.e. two days underway by land or sea equivalent to 85 US miles), if they surpass the homes of their village or tents of their people.Though shortening is better, completion is not disliked. Repeating is not required of anyone who returns before reaching the distance. Whoever intends unlimited residence in a place, residence for more than four days, or follows a resident, is to complete it. Perpetually shortening is legislated due to being withheld wrongfully, due to rain, or staying to handle a need (without the intention to reside for more than four days),—while not knowing when it will end.
It is permitted to combine Ẓuhr with ʿAṣr and Maghrib with ʿIshāʾ in either of their times. It is preferred to avoid it, except for the gatherings of ʿArafah and Muzdalifah, which is then recommended.
Combining can be done in the following eight cases: traveling the distance in which shortening is allowed; the ill who face difficulty if not combined; a wet-nurse due to difficulty from abundant filth; pseudo-menstruation etc.; anyone incapable of purifying themselves or making tayammum for every prayer; not knowing the time, e.g. the blind etc.; or due to some excuse or work which permits abandoning Friday and congregational prayers.
Combining between Maghrib and ʿIshāʾ is specifically permitted (even if prayed at home) due to snow, cold, ice, mud, strong cold wind, and rain which soaks the clothes, coupled with difficulty.
In regard to advancing or delaying, the best is that which is easiest. If they are equal, then delaying is preferred.
Sequence is an absolute prerequisite for combining. The following are prerequisites for combining during the time of the first prayer: the intention to combine before the iḥrām of the first; not separating the second (except with the equivalent of the iqāmah or a light wuḍūʿ), which would thus be invalided by an emphasized routine prayer between them; the presence of an excuse at the beginning of both and the first salām; its continuity (except when combining for rain etc.) until the completion of the second. Therefore, it is not invalidated if someone makes iḥrām for the first because of rain, which ceases and does not continue though it produces mud. If it does not, it is.
Additionally, if travel concludes with the first prayer, the combining and shortening are invalidated. It is, however, to be completed and is valid as an obligatory prayer. If travel is concluded during the second, they are invalid, but it is to be completed as a voluntary prayer.
The only prerequisites to combine in the second’s time are to have the intention from the first’s time (so long as there is still time to make it) and the presence of the excuse at the time of the second.
It is not a prerequisite of validity to have only one Imām. It is, therefore, valid if the follower prays: both behind two Imāms; behind someone not combining; one alone and the other in congregation; leading one in the first and another in the second; or with someone not combining.
More on shortening can be viewed HERE.
Source: Bidayat al-Abid