What type of marble do you have.

The type of marble is very important. Let us know. In the mean time perform a water test to check if the stone was sealed ad how well t resists water infiltration.

How often you have to re-seal depend on a few things such as use frequency, types of body products used, and cleaning agents used.

Sorry for the late reply. One bathroom has UN-HONED Botticino shower shelves & window ledge. (Tub, so no shower saddle.) I would have liked it to be honed throughout, but I didn't think about it till it was already installed. After I realized it, I got a HONED Botticino main-door saddle for this room.

In the other bathroom:
HONED Carrara for shower shelves, window ledge, shower saddle, and main-door saddle.

Thanks!

Botticino is great stone-its quite dense.
Therefore even honed it won't take in much sealer.
Not sure what you mean by unhoned-polished?
The carrara is another story as it can be very porous.
White marbles can also have ferrous minerals in their composition.
Placed in wet environments they can oxidize or rust.
Sometimes white marbles will exhibit yellow,brown or orange stains.
White Carrara when it gets wet looks very grey.
It is one of the marbles I think should be sealed in a shower stall.
I am not a fan of sealing all marbles in a shower stall because I think in some cases water getting below the stone will take longer to evaporate because of the sealer.
To answer your questions .
How often to seal -it isn't the sealer that will protect your stone as you have been lead to believe . It is the installation along with the proper specification of material.
Natural stone requires in most cases impregnating sealers however some slates and other products can be top coated .
Topcoats are acrylics and waxes which will need to be maintained and will fail often in shower stalls.
The performance of an impregnating sealer is to temporarily inhibit the intrusion of a staining agent.
When you think about that -how many things in the shower can stain the stone. The culprit is usually water and not from above the stone but from below it.
This is why caulking all the change of plane joints is so important. Along with the proper slope to the drain and the proper construction of the shower pan. Even then there can be some issues.
Polished marbles in shower stalls will over time become uniformly honed by steam. Something that isn't talked about often.
I hope this helps.

Srosen, thank you for your thorough, informative, helpful reply. What I meant to say is that the Cararra is HONED. The Botticino (except for the saddle) is not honed.

In the Cararra bathroom, I will only be washing hands/face and showering. I will have only mild gentle agents in that room. No toothpaste, cosmetics, not even shampoo. Probably just Dove body wash and an equally mild hand soap. (And sink is not near any of the marble.)

I will be applying makeup and shampooing in the Botticino room, but even there, the sink is nowhere near the marble. And I will make sure that whatever bottles I put on the shelves and/or their contents won't stain them. But from what you said, I am now less concerned about these shelves than I am the Cararra.

Thank you again.

You are the BEST!

Laura