card statement).
a statement period is usually 30 days ? so if you have a 55 day interest free period on your card, you will have an extra 25 days from the end of your statement period to pay the full balance before interest is charged (that is a total of 55 days).
so you do not have a flat 55 days interest free on all purchases, the interest free period will in fact be more or less on a particular purchase depending on when in the month you buy.
not paying off the balance each month?
the interest free period only applies where you pay your balance off in full at the end 