Well assuming you are doing the 60 gal throughout the day evenly, that is 2.5 gal an hour, well under the .5 GPM they recommend so that'll help the time it'll last.
That said, 3500 gallons at 60 gallons a day is a hair over 58 days.
That said, 3500 gallons at 60 gallons a day is a hair over 58 days.