Common phrases regarding shop or service availability:
Written: Available 24/7
Verbal: Available twenty four seven
Written: Available 24hrs
Verbal: Available twenty four hours
Written: Open 7 Days
Verbal: Open seven days
Written: Open 7 Days a week
Verbal: Open seven days a week
Written: Open 365 days
Verbal: Open three hundred and sixty five days / Open three sixty five days
Written: Open 365 days a year
Verbal: Open three hundred and sixty five days a year
I haven't seen the term 7/7 used, but given that its form is similar to 24/7 I'd suggest that 'seven seven' should be used.
I would suggest that 7 days would be a better term to use over 7/7 though - it's more explicit that it's referring to days a week and it's one less syllable. I don't see what 7/7 confers over 7 days or 24/7, except for being a cutesy variant of 24/7.