Mine lasted for about a month.... yeah, looks disgusting, but everyone goes through it. What really helped clear up the remnants in my tank were astrea snails. They don't do much for the sand bed, but they will scour the glass & rock. For the sand bed, whenever I did pwc
's, I just lightly skimmed a gravel-vac across the surface of the sand and sucked up a bit at a time..... don't try to do it all at once.
And don't worry if it occasionally comes back.... even many-year old tanks can have outbreaks. Sand stirrers can bring up fresh silicates, and it will have to run it's course again.
And just to be sure.... you are using RODI
water, right?? if not, you can continually feed diatoms with tap water.