The average price for a pound of butter in 1975 was $1.025.
The average gross hourly wage for a factory worker in 1975 was $4.81.
4.81/1.025=4.692 lb/hr
60/4.692=12.78 minutes to buy one pound of butter.
Sources:
Avg Wage (see chart 606): https://babel.hathitrust.org/cgi/pt?id=mdp.39015021301612&seq=406
Avg Prices:
https://babel.hathitrust.org/cgi/pt?id=mdp.39015008856711&seq=527