you should always sanitize fresh.
If not you are playing Russian Roulette with you beer, eventually it will bite you in the butt.
You should sanitize on bottling day (or brew day if you are brewing.) It only takes a few minutes. If you let a no-rinse, wet contact sanitizer like starsan or iodophor, dry your are reducing it's efficacy by half. If it is dry, any micro organisms that touch the surface render it no longer sanitized. If the walls are wet with sanitizer, that organism would be toast. But dry it would still be alive.
It really should take you no more than 10 minutes to sanitize your two cases of beer. Or bottle in an hour. If it takes you longer you need to work on your process, not cut corners with your sanitization regimen.
Here's a lot of bottling tips to make the job easier. http://www.homebrewtalk.com/f35/bott...ebrewer-94812/
You'll find a lot of good info/tips to effective sanitization here; http://www.homebrewtalk.com/f39/sani...uestion-54932/
in cluding how you should get rid of your dry powdered "pseudo sanitizers" in favor of the afore mentioned, no-rinse, wet contact sanitizers like starsan and iodophor.