Can i use my propane tank and regulator from my grill to run a propane furnace and how long would a 20# last ?

Sure you can ...the only thing you need to do is have your lawyer put me as your sole benificiary to your life insurance policy.....no do not do this ...there is a reason they have special valves for furnaces....call a pro .. the issue is not the tank ,but the regulator. there is no safety for failure...please please listen and get a pro to do it