General Motors is spending more than $2 billion in cash a month and lobbying for a government bailout to keep it out of bankruptcy. And for about 100,000 of its white-collar retirees, time is about to run out on GM's gold-plated medical benefits. To conserve its dwindling cash reserves, G.M. is eliminating lifetime healthcare coverage for its legions of retirees at the end of this year. After that, former employees will be left to fend for themselves in deciding how to cover their doctor's bills and prescription drug costs.