70% is not something you will know until you put the buds in jars/roasting bags with a hygrometer, but the time to move them off the lines/rack is when the stems break internally but don't crack in half when you bend them. How long it takes to get there depends entirely on the relative humidity wherever you are drying them.how do you know when they are 70% dry? I have read that the stems should snap and buds should sound crunchy like popcorn and it usually takes 7 to 10 days given the right humidity?
7-10 days is great if you can control the RH, but you take what you can get. If they get dry in 3 days, then you move them to jars/bags with a hygrometer in them and see where it settles after 12 hours. Over 70%? Take them out and let them sit in the dark another day. 70% or below? Start burping them twice a day!