Hi Jon and welcome to the forum.
This is a bit of a how long is a piece of string question to answer.
There are many factors that will determine TDS out. Such as.. TDS in.. Water pressure.. temperature etc.
The Merlin likes decent water pressure (60 PSI plus) Much lower and it is pretty inefficient. It is fine with good water pressure and a good idea is to add a carbon prefilter to pass the water through before the Merlin. Some also add a softener unit as well.
These factors will aslo affect the output rate so how long to fill a tank is a bit hard to give an answer to. I am sure some merlin users will come along soon with their own experience to help you though
Cheers
Andrew