I voted 6, but only because there was no better, not-too-much-and-not-too-little option.
Personally, if not adding custom feats to the mix (as I do with Improved, Greater, Superior, and Uncanny Toughness) that improve copies of Toughness feats, I'd say the feat needs to scale automatically.
Say it yields 4 or 5 HP when first taken, and says "plus 1 hit point per three hit dice or levels you possess. As your hit dice or levels increase, so do the hit points from this feat."
Maybe instead yield +1 HP/4 HD instead for the scaling, but I still figure 4 HP to be a good number for low-level acquisition. (a human wizard could take two copies and gain +8 HP at 1st-level, for example, but that'd just put him on-par with a 1st-level barbarian of equal Constitution)
If Toughness were only available once, rather than multiple times, then I would probably figure it to be worth 5 or 6 hit points, plus an additional 1 HP per 3, 4, or 5 levels.
In some of my games, I houserule Toughness to grant 4 HP instead of 3, but I also have one or more other feats that require it and improve upon it. It remains some use for the first few levels, but beyond that you'd need to take one or two of the other Toughness feats to make it worthwhile. I use Uncanny Toughness in Rhunaria, and a chain of Improved, Greater, and Superior Toughness in Aurelia. UT requires two copies of Toughness, gives 5 HP, and makes all other Toughness feats yield an extra 2 HP (making them all give 5 HP each, since I don't change Toughness itself in Rhunaria). For Aurelia, Toughness yields 4 HP, Improved T. yields 8 (and requires 2 copies of Toughness) and is only available once, Greater T. provides 3 HP for each feat possessed with "Toughness" in its name (requiring 2 Toughness feats and 1 Imp. T.) and is also available only once, and Superior T. yields 1 HP per hit die or level (retroactively and henceforth) plus 1 HP per previous feat gained with "Toughness" in the name (and requires all four previous T. feats).