In my understanding, Rule 43 of the CCS(Leave) Rules governs the issue:
43. Maternity Leave
(1) A female Government servant (including an apprentice) with less
than two surviving children may be granted maternity leave by an
authority competent to grant leave for a period of (135 days) from
the date of its commencement.
(2) During such period, she shall be paid leave salary equal to the pay
drawn immediately before proceeding on leave.
NOTE :- In the case of a person to whom Employees’ State Insurance
Act, 1948 (34 of 1948), applies, the amount of leave salary payable under
this rule shall be reduced by the amount of benefit payable under the said
Act for the corresponding period.
(3) Maternity leave not exceeding 45 days may also be granted to a
female Government servant (irrespective of the number of surviving
children) during the entire service of that female Government in case
of miscarriage including abortion on production of medical
certificate as laid down in Rule 19:
`Provided that the maternity leave granted and availed of before the
commencement of the CCS(Leave) Amendment Rules, 1995, shall not be
taken into account for the purpose of this sub-rule.
(4) (a) Maternity leave may be combined with leave of any other kind.
(b) Notwithstanding the requirement of production of medical
certificate contained in sub-rule (1) of Rule 30 or sub-rule (1) of Rule
31, leave of the kind due and admissible (including commuted leave for
a period not exceeding 60 days and leave not due) up to a maximum of
one year may, if applied for, be granted in continuation of maternity
leave granted under sub-rule (1).
(5) Maternity leave shall not be debited against the leave account.
As could be seen, there is no minimum eligibility period. However, the facility available only if you are in service. It is quite possible for you to join the service and avail of maternity leave immediately thereafter, if you are so eligible.