I believe an absence of longer than 6 months can disrupt the period required to naturalize, unless this has changed. My understanding of the following excerpt is that if you leave for more than 6 months, even if you haven't abandoned your permanent residence, the 5 or 3 years or residence required to naturalize would start at zero again. But someone please clarify if they know better! Thanks.
PART 316--GENERAL REQUIREMENTS FOR NATURALIZATION
c) Disruption of continuity of residence--(1) Absence from the
United States--(i) For continuous periods of between six (6) months and
one (1) year.
Absences from the United States for continuous periods of
between six (6) months and one (1) year during the periods for which
continuous residence is required under Sec. 316.2 (a)(3) and (a)(6)
shall disrupt the continuity of such residence for purposes of this part
unless the applicant can establish otherwise to the satisfaction of the
Service. This finding remains valid even if the applicant did not apply
for or otherwise request a nonresident classification for tax purposes,
did not document an abandonment of lawful permanent resident status, and
is still considered a lawful permanent resident under immigration laws.
The types of documentation which may establish that the applicant did
not disrupt the continuity of his or her residence in the United States
during an extended absence include, but are not limited to, evidence
that during the absence:
(A) The applicant did not terminate his or her employment in the
United States;
( B ) The applicant's immediate family remained in the United States;
( C) The applicant retained full access to his or her United States
abode; or
(D) The applicant did not obtain employment while abroad.
Source:
http://tinyurl.com/y56fefI also found this explanation on westlaw (I have access b/c I'm a law student):
An applicant for naturalization who is absent for six months to a year is deemed to have abandoned lawful permanent residence.
Absences from the United States for continuous periods of between six months and one year during the periods for which continuous residence is required breaks the continuity of such residence, and leads to the conclusion that the applicant has abandoned lawful permanent residence in the United States for naturalization purposes, unless the applicant can establish otherwise. This conclusion remains valid even if the applicant proves that he or she did not apply for, or otherwise request, a nonresident classification for tax purposes, that he or she did not document an abandonment of lawful permanent resident status, and that he or she is still considered a lawful permanent resident under immigration laws. The types of documentation which may establish that the applicant did not abandon his or her lawful permanent residence in the United States during an extended absence include, but are not limited to, evidence that during the absence the applicant did not terminate his or her employment in the United States; the applicant's immediate family remained in the United States; the applicant retained full access to his or her United States abode; or the applicant did not obtain employment while abroad.
SO, it looks like you could be deemed to have abandoned PR status as far as naturalization goes, but would not actually lose your PR status. This may be way more than you wanted to know, sorry

But I hope it helps.