I can't answer your question, but I do have a comment. Do you want this bath remodel to be timeless, or do you want something that in twenty years people will be able to pinpoint the decade when it was redone? This is a pretty trendy application in my opinion. Take this into account too when you do your remodel.

I would contact Walker Zanger with your question. They know their slate well and will be able to give you the best answer as not all slate will wear the same, meaning some may need sealing more frequently, others less. WZ may suggest a specific sealer etc...

It's "one of those things".

Glass typically does not need a sealer. But glass tiles with decorative finishes can actually have slight fissures or a texture to that decorative finish, and that surface finish can benefit from a sealer.

Somewhat the same with slate. A true high-quality HARD slate is a thing to behold. The waterline tile in my swimming pool is slate. It's survived well over a decade of winter freeze/thaw cycles with no water absorption and without the resulting spalling or pitting that would have ruined a more porous stone. However...

"Slate" can be many things these days. A honed slate will usually originate from a better quality and harder slate than a some cleft slate because some softer slates are too soft to survive the honing process. They'd disintegrate in the process, so they get marketed as cleft. That's not an indictment of all cleft slate. Just the soft flaky ones.

If the manufacturer says to seal your tile, then seal it. As to how often it needs to be resealed, you'll pretty much get a generic "reseal it whenever is needs resealing" answer. When you first seal the slate, you may see water beading on the slate. Over time the water may eventually just create wet marks on the slate instead of beading up, or the wall tile may appear wet several hours and after a shower where it dried faster in earlier days. That might be a good time to let the shower fully dry and then reseal.

A good quality impregnating sealer should perform well for years. Two years, three years, five years even. It's the cleaning methods that can mess things up. If you use shower products (soaps, lotions and potions) that gunk up your tile and grout with lotion/potion residue, you'll have to clean and scrub more often. When you do have to clean, if you use a cleaner that is NOT pH neutral, one that is more harsh on the grout and tile and sealer? Then you'll have to seal more often.

If you seal with a good impregnating sealer...

If you don't use harsh cleaning products...

Then you might only have to reseal every 3-5 years.

I'd ask W-Z what sealer they recommend. I'd follow the guidelines of that sealer.

So overall, a somewhat ambiguous answer. "It depends...".

Report back if you get any added info or of you have any other Q's.