How many attacks when TWF?

[Lawrence DuBois]

So this is a pretty basic question, but I just never thought about it until now. I never really got to high-level play in a party with any TWF characters.
Reading over the TWF rules and feats, I've come up with a few ambiguities that I just always assumed I knew the answer to simply because I never thought about them. I think I can figure out the answers with RAI, but it's a bit awkward given that I can't really find anything resembling (explicit) RAW on the subject.

First, I always assumed that a typical, standard-action attack allowed the off-hand attack. I'd like to make sure I'm not wrong on that, first. So I guess it would follow that with Greater TWF a standard attack lets you attack once with your main hand and thrice with your off-hand. Is that so? Or do you have to make a full attack in order to use TWF?

Then, if your a 20th-level fighter and you have Greater TWF, does that mean you get a total of 7 attacks when you make a full attack? Or do the extra off-hand attacks usurp your usual extra attacks.

Honestly, the 7 attacks solution seems to make the most sense given that you can get TWF before your BAB allows a second attack, I just never thought you'd be able to make so many in one turn before, so it's surprising me.

[Calth]

TWF only works with full-attacks, you never gain off-hand attacks with the standard attack action.

A character with 16+ BAB and Improved TWF and Greater TWF would have 7 attacks while using TWF during a full-attack.

[Corvino]

You have to full attack in order to benefit from TWF, stated in the Full Attack section of the combat rules. No off-hand attacks on AoOs, Charges or Standard Attacks.

[Lawrence DuBois]

Okay. Thanks for that. Never occurred to me to look in the full attack rules. ^^'

[Claxon]

Excluding the Two Weapon Warrior fighter archetype (and possibly others that I may not be aware of) you only get to make 1 attack unless you use the full attack action to TWF.

So on a standard action you normally only get 1 attack.

A 20th level fighter would have 4 attacks from high BAB (1 regular and 3 iterative). TWF allows you to make 1 extra attack (though the feat itself actually only reduces the penalties you can TWF without the feat but it's a terrible idea). You must take Improved Two Weapon Fighting to gain the 2nd off hand attack, and Greater for the 3rd off hand. This would result in a total of 7 possible attacks.