As what i have seen on the coinmarketcap.com for the market price for bitcoin that literally goes down to 6K usd is a good time to start the investment. I am waiting for this moment but still i need more money to invest on it. Well i hope i could get more money the following day so that i could invest in this time where bitcoin market price falls down.
I'm not sure what else you need to wait for the decrease in bitcoin, I think now is the time to buy, but this is my opinion. How much you want to invest is for you to decide