Well, I can start by saying it will vary from tank to tank. I have swords, vals, crypts, and dwarf sag in my tank. They show no real deficiencies (I have leafs die and whatnot, but not whole plants). My tank hasn't slowed it's growth at all so I haven't added more root tabs in since it started. My tank is low tech, so your mileage may vary. But the age old adage of "don't fix what isn't broken" has worked for my tank. I haven't needed to replace root tabs, so I haven't.
As always, I could be misinformed, remembering incorrectly, or be an outlier myself.
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