It may be weeks, even months, before federal workers, postal employes and retirees get a chance to shop for better or less expensive health insurance even though the average premium will jump 30 percent next month.

Many plans will cut back benefits in January, and most will increase biweekly premiums paid by employes and monthly premiums charged to retirees. But it could be some time before the Office of Personnel Management holds -- or is ordered to hold -- an open season enrollment period when people can make changes in their coverage.

Earlier this month OPM won a stay (of up to 30 days) in a lower court order which said the federal personnel agency had to hold an immediate open season. That stay expires in early January, but OPM is planning to ask for an extension, on grounds that it will need time -- at least 6 weeks -- to print and distribute new brochures listing benefit changes in each of the 120-plus plans in the program, and to advise people of premium changes.

Federal unions and many of the carriers hope the court will order OPM to hold the open season sooner. But nobody knows if or when that will happen.

A three-judge Court of Appeals panel here yesterday upheld a 6.5 percent across-the-board cut in benefits OPM had imposed on most of the health plans last October. National Federation of Federal Employes, the union which went into court arguing that OPM could not mandate the cuts, says it will appeal the three-judge ruling to the full court. Meantime, OPM must reenter negotiations over benefits with many of the carriers, and all this will take time.)