Intercept angles (radial / localizer / radar vectors)

Typical practical intercept angle to acquire a VOR radial or airway: about 30°–45°, adjusted for closing speed and distance to the fix.
Procedure design (PANS-OPS) generally keeps the intercept angle onto the final approach course at 30° or less for a stabilized intercept near the final approach fix; larger angles are only used further out, on initial/intermediate segments.
For an ILS localizer, an intercept angle much greater than about 30° close to the runway makes it difficult to establish before the glideslope/final approach fix — ATC radar vectors are normally planned within this guidance.
These are design and technique values, not one fixed legal number in every case — always fly the angle implied by the published procedure or the actual ATC vector given, not a rule of thumb, when the two differ.

Source: ICAO Doc 8168 (PANS-OPS) – Instrument approach procedure construction

IFR