Does one have to do ijtihad of the prayer time and qibla direction for every prayer? Can one follow a timetable, and if so, does one have to check it every time, or is it permissible to rely on one’s memory of the previous day’s time, for example.
In residential settings once suffices for as long as one resides there. As for prayer times in the absence of adhan one needs to be sure, and if memory affords certainty then it suffices, if not then one needs to check. In travel through desert lands or non-Muslim urban settings devoid of mosques and adhan, one needs to do everything mentioned for each prayer as described in detail in Imam Ghazali’s “Book of Travel and Travellers” which is explained in full here:
The parts on obligatory ijtihad in travel are from Class 23 to the end.
Hajj Gibril Haddad