It will depend on the thickness or gauge of the wool. Find out what kind of wool you want to use, that will also determine the size of your needles.

In general, thicker the wool, fewer the rows.

It really depends of the size of the yarn and the size of the needles you are using as to how many rows in a scarf.
Just have fun and make the scarf as long as you want it be!
Because a pattern says it should be 4 ft., doen’t mean it has to be 4 ft. Make the scarf your own and stop knitting when YOU think it is finished

You could measure your scarf when you reach 12 inches. Then count the number of rows it took to get that far.
Take that number and times it by 4.
That amount should be the total number of rows it will take you to get to 4 feet.

Instead of doing a scarf that is 40 stitches wide and about 300 rows long, make one that is about 360 stitches wide and only about 12 rows long. Use 2 strands of worsted weight yarn and a size 13 needle. It’ll work up very quickly!

If you know the length, why do you need the number of rows? I never use that count, but always go by measurements. I ofttimes do need to know how many stitches wide to make something (so I know how many to cast on) but then I just keep knitting until I have the right length.

it will depend on the thickness of the scarf

Look at the wrapper of the yarn you want to use, you should find a square that tells you how many stitches and how many rows there are for that yarn on the needles recommended in either 4″ or 10 cm (the same thing). Multiply the number of rows in this gauge by 12 to determine approximately how many rows will need to be worked. Your actual count may differ depending on your actual working gauge.

if you want a scarf that is 4 feet or 48 inches long then you need to know your row gauge. lets say that its 7 rows per inch, then 7*48=336 rows. make sure that you count even a quarter of a stitch, this will make a big difference

check this link to figure out what gauge is