The max exp you need for 1 paragon level is 2,1~ billion. Today you can do up to 2 billion/h. Minus the 50% buff it will be around 1 billion/h. So 2h per paragon max :P
https://docs.google.com/spreadsheet/...=sharing#gid=0
Level 1488 requires 2.147.483.647 exp and it doesn't get higher beyond that point.