ISS flight rules have not been published publicly AFAIK. Here are the Shuttle flight rules concerning EVA scheduling.
Sorry for the "yelling" but that is how the document is written. The rules are in all caps and bold. The rationale is in italics.
- A "scheduled EVA" is any EVA incorporated into the nominal flight
plan. [interpretation - an EVA specifically planned and trained for as part of the mission]
- An "unscheduled EVA" is used to achieve/enhance the mission
objectives of the orbiter or its payloads. It is not part of the
flight plan. [interpretation - an EVA being carried out to resolve an issue, not specifically planned and trained for. May be covered by generic training]
(Defined in rules A15-4 and A15-5)
A15-14 SCHEDULED AND UNSCHEDULED EVA CONSTRAINTS
A. NO SCHEDULED EVA WILL BE PERFORMED PRIOR TO MET 72 HOURS.
During adaptation to zero-g
conditions, moving about may provoke symptoms of illness. The activity
associated with putting on a suit would increase the chance of an EVA
crewmember being ill and potentially endanger the crewmember with
vomitus in the suit.
B. UNSCHEDULED EVA’S:
- NO UNSCHEDULED EVA SHALL OCCUR DURING THE FIRST 24 HOURS OF FLIGHT.
- AN UNSCHEDULED EVA MAY BE PERFORMED PRIOR TO MET 72 HOURS ONLY IF AN ASSESSMENT OF CREW HEALTH HAS BEEN ACCOMPLISHED BY THE FCR SURGEON
AND APPROVED BY THE FLIGHT DIRECTOR.
Asymptomatic crewmembers may be
able to safely perform an unscheduled EVA before MET 72 hours. An
accurate assessment of the health status of the EVA crew can be made
by the FCR surgeon during a private medical conference.
C. NO SCHEDULED OR UNSCHEDULED EVA SHALL BE PERFORMED ON ENTRY DAY.
FOR A FLIGHT USING A SINGLE CREW SHIFT, A SCHEDULED OR UNSCHEDULED EVA MAY BE PERFORMED ON THE DAY BEFORE ENTRY ONLY IF AIRLOCK INGRESS
IS COMPLETED NOT LATER THAN CREW SLEEP MINUS 7 HOURS.
FOR A FLIGHT USING DUAL SHIFT OPERATIONS, A SCHEDULED OR UNSCHEDULED EVA MAY BE PERFORMED ON THE DAY BEFORE ENTRY ONLY IF
AIRLOCK INGRESS IS COMPLETED NOT LATER THAN CREW SLEEP (NONENTRY TEAM)
MINUS 4.5 HOURS.
An EVA may be done on the day before entry if there
is adequate time post-EVA to perform required activities. These
activities include airlock repress, post-EVA, EMU maintenance
recharge, post-EVA entry prep, cabin stow, and presleep. Seven hours
is considered the minimum amount of time to accomplish these
activities for a single shift flight. FCS C/O, RCS hot fire, and cabin
repress activities must also be performed but can be overlapped with
post-EVA and cabin stow activities. For an unscheduled EVA on the day
before entry, it will be necessary to perform this EVA as a “quick
response” EVA (i.e., the EMU C/O and cabin depress will need to be
done on the day prior to the EVA). For a dual shift flight, 4.5 hours
is considered the minimum due to cabin stow occurring on entry day for
the nonentry team.
Source (huge pdf)
EVA - Extravehicular Activity (spacewalk)
MET - Mission Elapsed Time
FCR - Flight Control Room (Mission Control Center)
EMU - Extravehicular Mobility Unit (spacesuit)
FCS C/O - Flight Control System Checkout
(activity performed prior to entry to ensure the orbiter is in good
shape for atmospheric flight)
RCS - Reaction Control System