Playing is something very important for in-door cats. They need a lot of exercises to compensate for living between four walls. Cats who don't run and play enough become fat and are predisposed to health problems. So ... how much time to spend playing with my cat? It depends on their personality and on some other factors. If cat lives with other cats it will be more likely to prefer playing with them not with a human. If it is that sort of a situation you won't have much work - just sit quietly and laugh. But if your cat is the only pet - it is time for a hard work. Throwing a small ball or dragging a toy around. And if your cat is already fat and lazy it will be very tiering. But don't give up so easy. After a week or too of fruitless jumping and crying your cat will pity you and will start to play!
Be careful and don't give a cat thread because it can be swallowed and can cause deadly problems. Cat toys and small balls are the best choice.