Once a week, try to increase things just a tad but understand the terrain you are going to ride. For example, my wife went on a 5 miler at first and we sloely increased from there with speed, distance and hills. She now can do 28 miles with rolling hills or 40 flat at about a 14 mph clip or 25-30 miler doing a 15 mph clip. She has come a long way.
Key is not to push her. Heck, my wife will still sleep several hours after the ride because she is tired.
Make sure she hydrates and eats properly. My wife is still figuring this out.
As far as how long it will take her... I have no clue. Dont overdo it. Thats all I know!