@momof5boys: I wasn't sure if you were asking Granger as you used my quote in your post, but I did mean to respond.
It depends on your starting point. I actually only use a hard toothbrush wet down with water and dish detergent. If you take care of your shower daily, you can actually keep in looking new for decades. My sister's bathroom grout was immaculate until the day she sold her house, so it's possible.
If there is black mold that has seeped into the grout and is discolouring it from the inside out, there's no remedy to get it white unless you rip out all the tiles and start again. Bleaching agents cannot soak into the pores because the walls are vertical and everything just runs down. My brother had some luck with spraying with a bleach solution and leaving it on for as long as he could (wiping down the shower as dry as he could, then sprayed on the solution and left it until his next shower, then repeat).
If there is discolouration, the bleach solution will work to some extent as well. You should open the window or turn on the fan for a little while to get the smell out if you have small children or pets.
In these cases, prevention is worth a pound of cure. If it's a new bathroom, you can keep it in pristine condition simply by wiping down the walls with a squeegee, then following up with an absorbent sponge after each shower (but not if people time the showers back to back). If things ever start to get a little brown, scrub with a hard brush and the stains should come out. Rinse well, squeegee, sponge down. DH isn't as thorough as I am, so I time my showers to be right after his.
Our home is a resale, so we have icky mold behind the tile from the last two families. It's one of the next reno projects within the upcoming decade and I'll be picking out some large tiles. Those equal less grout to scrub and clean, which saves a lot of time.
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Thanks sweetsparrow. My hubby and my shower is in pristine condition because I wipe it down often. It is my boys shower that doesn't get wiped down regularly and one of them is a mechanic so that explains why some of the grout is discolored. It doesn't look terrible but I'm a little on the fussy side :-) I will try a toothbrush with Oxyclean but would rather not use a bleach solution because it could weaken the grout.