Those are the signs of SP setting in. How long is impossible to say. What exactly do you mean when you say that you couldn't concentrate once your heart started beating loudly? I'm thinkinking that you were on the verge but for one reason or another opted out at the last minute. It could be that the reality check snapped you out of it. IMO a reality check isn't really nessesary at a time like this. You will either see the imagery building and you will slip into the dream, or you will head off into an OBE and separate from your physical body. Either way a reality check doesn't really apply.