As he swims Jason counts the number of lengths he has swum so far and converts this number into a fraction of the whole day's swim.

Interestingly he finds that, after the first length, the only numbers of lengths that will not cancel down are prime numbers.

(If, for example, he always swam 15 lengths, this would not be the case, because 8 is not a prime but 8/15 cannot be cancelled down.)

Jason swims the highest number of lengths for which this is possible.

How far does he swim each week?