Just for an example:
An iPhone 6 can be charged to 90% in two hours with an 2.1A power adapter. I would not try to reduce the time with higher currents due to thermal issues. A 45W panel has a nominal output current of around 2.1A at an area of roughly two by four feet. A 20W panel has roughly 1.1A nominal output current at an area of roughly two by two feet.
The charging times will depend on how far you deplete the battery.
The actual output depends on the panels technology and may differ from numbers above. It still should give you an idea on how practical it is.