New Marcos Rock to established reef tank
 

Looking to add some additional rock to the tank but not adding any “live rock” due to past experience with pest, so got me some Marcos Rock.

Since there been threads about it leaching phosphates, I’m soaking it in RO/DI and changing water every other day.

Any advice on how long I should do this to get most of the phosphates out?

2 to 3 weeks

Before you waste all that water making changes, I would test the water your rock is soaking in for phosphates. If there are not phosphates in the water the rocks are soaking in there is no need to make water changes. I've used Marco Rock in several aquariums and never had an issues with their rock containing phosphates although like you I initially soaked the rock in RO/DI water, but it never tested with phosphate,

It should have almost none in it, its rock from deep under the surface, similar to his sand.

I'm almost a decade in with his sand, no issues with phos at all

We can't answer this question reliably. Test the water. If the phosphate level is higher than you want, continue with water changes or consider a more active method like dosing LC.

Some folks have few issues and some have several years worth of issues... all from the same place. You will not know until you get them.

If they have phosphate, they will be bound to the core and can take a long time to fully remove. Acid bath of freshwater soak can get some on the surface, but it won't get all of them.