Since you're referring to ACA, I assume you're talking about 2014, because the provisions you're referring to (paying a penalty) doesn't go into effect until then. The individual mandate allows for a gap of three months. Courtesy of BCBS:
Beginning January 1, 2014, all U.S. residents are required to maintain minimum essential
coverage unless the individual falls into one of the following exceptions:
individuals with a religious conscience exemption (applies only to certain faiths);
incarcerated individuals;
undocumented aliens;
individuals who cannot afford coverage (i.e. required contribution exceeds 8% of
household income);
individuals with a coverage gap of less than 3 months;
individuals in a hardship situation (as defined by the Secretary of Dept. of Health &
Human Services (HHS));
individuals with income below the tax filing threshold; and
members of Indian tribes.
(PPACA §§1501 and 10106 adding IRC §§ 5000A(d) and (e); §1002 of Reconciliation Bill)
Of course, be aware that you're not covered during that gap (except by COBRA, implicitly, but only for the latency period.)