HOW LONG CAN Prince George's County live on TRIM alone, before its taxpayers decide more money has to come from somewhere? The TRIM amendment, which has maintained a flat dollar ceiling on revenues from property taxes, keeps surviving voter tests, even if support for some modification may be growing. But it doesn't take a certified public accountant to figure out that the costs of county government--cuts or not--are increasing. Where is the money to come from?

Annapolis was the first stop. There, county executive Parris Glendening--who campaigned hard and unsuccessfully for a modification of TRIM on his way to election--ran into predictable reluctance on the part of state legislators to bail out a county that had refused to shell out. Then and still, the budget pressures continue: county employ- ees are enduring pay envelopes without cost-of-living increases, and services are less than what they were.

If a majority of residents in the county still wants to go on living this way, so be it. But those who don't may wish to support new attempts by Mr. Glendening and other county and state officials to raise revenues. For example, there may at last be an active constituency for a more equitable restructuring of the state income tax--a change long overdue, never politically timely and usually given lip service and nothing more. Gov. Harry Hughes has indicated his support for such a proposal, and Prince George's residents--as much as any Marylanders-- should be eager for the change.

Even then, memories in Annapolis--unlike revenues in Prince George's--are not short, and no pot of gold will be sitting there for the county's taking. At some point, TRIM must self-destruct.