flyfly4,
PANS-ATM
6.5.7.1 An expected approach time shall be determined for an arriving aircraft that will be subjected to a delay of 10 minutes or more or such other period as has been determined by the appropriate authority. The expected approach time shall be transmitted to the aircraft as soon as practicable and preferably not later than at the commencement of its initial descent from cruising level. A revised expected approach time shall be transmitted to the aircraft without delay whenever it differs from that previously transmitted by 5 minutes or more, or such lesser period of time as has been established by the appropriate ATS authority or agreed between the ATS units concerned.
You will see there are two sorts of delay, an initial delay and a subsequent delay.
6.7.3.2.5 Subject to radar system and situation display capabilities, a minimum of 5.6 km (3.0 NM) radar separation shall be provided between aircraft on the same ILS localizer course or MLS final approach track unless
increased longitudinal separation is required due to wake turbulence or for other reasons.
Note 1.— See Chapter 8, 8.7.3.4.
Note 2.— An aircraft established on an ILS localizer course or MLS final approach track is separated from another aircraft established on an adjacent parallel ILS localizer course or MLS final approach track provided neither aircraft penetrates the NTZ as depicted on the situation display.
but if you follow through the reference you find 3.0NM can be reduced to 2.5NM under some circumstances.
8.7.3 Separation minima based on ATS surveillance systems
8.7.3.1 Unless otherwise prescribed in accordance with 8.7.3.2 (with respect to radar), 8.7.3.3 or 8.7.3.4, or Chapter 6 (with respect to independent and dependent parallel approaches), the horizontal separation minimum based on
radar and/or ADS-B shall be 9.3 km (5.0 NM).
8.7.3.2 The radar separation minimum in 8.7.3.1 may, if so prescribed by the appropriate ATS authority, be reduced, but not below:
a) 5.6 km (3.0 NM) when radar capabilities at a given location so permit; and
b) 4.6 km (2.5 NM) between succeeding aircraft which are established on the same final approach track within 18.5 km (10 NM) of the runway end. A reduced separation minimum of 4.6 km (2.5 NM) may be applied,
provided:
i) the average runway occupancy time of landing aircraft is proven, by means such as data collection and statistical analysis and methods based on a theoretical model, not to exceed 50 seconds;
ii) braking action is reported as good and runway occupancy times are not adversely affected by runway contaminants such as slush, snow or ice;
iii) a radar system with appropriate azimuth and range resolution and an update rate of 5 seconds or less is used in combination with suitable radar displays; etc., etc.
You can find these documents online. Your groundschool should have them at least in electronic form and should be able to answer your queries.