When we say "how long, not long", it means that something is going to happen very soon or very quickly. It's like when we ask mom or dad, "are we there yet?" when we are in the car, and they say "not long now!" which means we will arrive at our destination soon.
Sometimes, we don't want to wait a long time for things to happen, like when we are waiting for our favorite TV show to start or when we are waiting for our turn to play a game. Saying "how long, not long" is a way to remind ourselves that we won't have to wait for too much longer.
So, if someone says "how long, not long" to you, it means that whatever you are waiting for is going to happen soon, and you won't have to wait for too much longer.