Pix, my friend, rarely do I find you mistaken and I think you are correct here too:
“The timer is about 30 minutes, if not mistaken (it’s an internal MS timer)”
See GSM 02.11 version 7.0.1 Release 1998 (ETSI TS 100 921 V7.0.1 (1999-07))
3.2.2.5 Timer for return to HPLMN
If the MS in Automatic Mode has selected and registered on a VPLMN of its home country, it shall make periodic attempts to return to its HPLMN.
The interval between attempts shall be stored in the SIM. Only the service provider shall be able to set the timer value. The timer shall have a value between 6 minutes and 8 hours, with a step size of 6 minutes. One value shall be designated to indicate that no periodic attempts shall be made.
In the absence of a permitted value in the SIM, or the SIM is phase 1 and therefore does not contain the datafield, then a default value of 30 minutes, shall be used by the MS.
NOTE: Use of values less than 30 minutes may result in excessive ME battery drain.