The "everyday" fractions -- two thirds, three fourths, seven eighths -- are written out when they stand alone. They are hyphenated only when they are direct adjectives.
...had an increase of three fourths over the prior year...
...one thirds of them were...
...had a one-fourth decrease in the rate...
...two-thirds majority...
If the fraction is "unusual" or large, use figures.
...was 21/1000 of...
...was 1/400 of an inch...
...had 17/64 drill bit...
If the fraction is with a whole number -- that is, a mixed number -- use figures.
...was 2 3/4 when I saw it...
...offered a rate of 3 1/3...
Happy punctuating!
Margie