You/they need to follow the directions for whatever sealer you elect to use. Seven days is best, but it's difficult to get people to wait that long.
Using the shower before it's sealed is not a good idea if you're planning to seal it. The water doesn't hurt, but all the other gunk that's gonna get on there does. And, again, you need to let the thing dry completely before you seal with most products, so follow the directions.
I've heard of some like Scott says that say you can use them much earlier, but I don't recall the brand. Nor do I know how effective they are.
I'll move this over to the Cleaning/Sealing forum to see maybe don't you get more input.
My opinion; worth price charged.
And please put a first name in your signature line for us.