Sometime next March, Time magazine will bring out a new program-listing magazine, TV-Cable Week. As its name implies it will cover both over-the-air and cable programming and will probably be offered by most cable systems for about 70 cents an issue. Some systems are already giving away free On Cable magazine, which concentrates on cable listings only. Meantime, ye olde faithful, TV Guide has added local cable listings in a desperate battle to maintain its almost monopolistic hold on the listings-magazine field. Only Time will tell.