If you lose your FEHB coverage because you leave your Federal job, you are eligible for TCC unless your separation is involuntary due to gross misconduct. Otherwise, the reasons for your separation don’t matter. Your Human Resources Office must notify you within 61 days after your regular FEHB enrollment terminates of your opportunity to enroll under TCC (temporary continuation of coverage). Generally, you have 60 days after getting the notice or 60 days after separation, whichever is later, to enroll under TCC. https://www.opm.gov/healthcare-insurance/healthcare/temporary-continuation-of-coverage/#url=Eligibility
In most instances, if you are on LWOP over a year the USPS will stop paying for your FEHB health benefits. This can also affect your getting health benefits in retirement due to the 5 year rule.
Even if you go over a year in LWOP status you have certain rights, one of those rights is to a Temporary Continuation of Coverage (TCC). This is similar to COBRA coverage but postal and federal workers are not eligible for COBRA but are eligible for TCC.