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.
And like Bend asked what all are you sanitizing? You only need to sanitize anything that is going to come into contact with your beer POST BOIL, meaning your fermenter, airlock, bung/lid, autosiphon if you are racking from kettle to fermenter, or funnel if you are pouring into a carboy, or any strainers you might be using if you strain, or you oxygen wand or stirrer if you use that to oxygenate before yeast pitch, your thermometer if you don't keep it in the kettle the whole time...And all I do for any of that stuff on brewday is fill my fermenter bucket with about 2.5 gallons or so of sanitizer, and swish/dunk stuff as needed, as well as using my spray bottle of starsan/RO water to contact sanitize as well.
You'll find a lot of good info/tips to effective sanitization here;
https://www.homebrewtalk.com/f39/sanitizer-question-54932/ .
