Here in Serbia, logs of a station must be kept for two years.
In the case of a club radio station, each operator must sign the log for every use of the station; however the case of a guest using the station doesn't seem to be explicitly defined.
The relevant document says the following (translated by me):
III.3. Log of an amateur radio station
The data relating to amateur radio communications of any amateur radio
station shall be entered in the logbook of the amateur radio station.
The log of an amateur radio station shall be kept in a manner that
provides a permanent record of written or electronic form.
Mandatory data to be entered in the logbook of amateur radio stations
- the day, month and year of actual amateur radio communication;
- start time of amateur radio communication, and in
case of longer transmissions, the start and end of the transmission;
- The call sign which identified emission correspondent amateur station;
- the name of the used amateur frequency band;
- the type of emission;
- the signature of the radio amateur (for club stations).
Day and month are entered as a group of four Arabic numbers.
The time is expressed as Coordinated Universal Time (UTC), and is
always entered as groups of four Arabic numerals marking the hours and
minutes (00 00).
In addition to the information specified in
paragraph 3 of this article, the information about signal quality and
reception quality, the location of the amateur radio station whose
transmission is received, the name of the operator of the
correspondent station, the type and intensity of disturbance, etc. can
The log of an amateur radio station shall be kept for at least two years
from the date of last entry.