It depends what exam cycle a particular loco is on.
A exams are typically at around evey week to 2 weeks, depending on the vehicle useage.
Beyond A exams it could be cumulative, e.g. B, C, B, D, B, C, B, E etc. Normally F exams and above are major 'works' type attention, and the highest exam is a K/INT which is pretty much what DBS are doing to the Super 60s.
Balanced exams, however, run numerically, B1, B2, B3 etc, and over the complete exam cycle, all the major work will be done in bits and bobs, meaning there is no need for the loco to be out of service for months having a major exam (F>). There is a big variation on how many exams in a cycle, for example 6 in the class 60 cycle, but 18 in the DBS class 66 exam cycle.
Items such as oil levels will get done regularly, such as every 2 days by maintenance staffs, and the drivers will also check levels as aprt of their perparation duties. Batteries get checked every exam (typically 6 monthly). Oil changes depends on the VMOI (maintenance instructions), but most diesel engines have a monthly oil sample taken, which is analysed by a Laboratory, and any exceptional readings are acted on. These are such as high sodium, indicating internal coolant leaks, high tin, pointing at big end or main bearing wear.