The 'iddah for a woman who has been given talaq is clearly defined in 2:228:
Divorced women remain in waiting for three periods, and it is not lawful for them to conceal what Allah has created in their wombs if they believe in Allah and the Last Day. ...
This is for the case of a woman who is in the age where she can have her menses. And this 'iddah starts from the day her husband spoke out talaq and it doesn't matter whether she was separated from him before the divorce or not.
See for example this fatwa, and the english translation here:
إذا كان الواقع كما ذكرتم، فلا ريب أن عليها العدة؛ لأن العدة لا تكون
إلا بعد الطلاق ولو طالت غيبة الزوج عن المطلقة؛ لقول الله سبحانه:
وَالْمُطَلَّقَاتُ يَتَرَبَّصْنَ بِأَنفُسِهِنَّ ثَلاَثَةَ قُرُوَءٍ
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked about a man
who stayed away from his wife and was travelling for twenty years,
after which he sent word to her of divorce. This woman wants to get
married – does she have to observe the ‘iddah? Because her husband has
been away from her for twenty years and has not touched her. Is the
purpose of the ‘iddah to establish that there is no pregnancy, or is
it for another purpose?
He replied: If the matter is as described, there is no doubt that she
must observe the ‘iddah, because the ‘iddah can only begin after a
divorce has taken place, even if the husband has been away from the
woman who is now divorced, because Allaah says (interpretation of the
“And divorced women shall wait (as regards their marriage) for three
So counting the 'iddah only starts after a talaq has been uttered not earlier than this.
For those women who may not have a menses (like old woman, young girls or pregnant women) there's also a predefined 'iddah (65:4):
And those who no longer expect menstruation among your women - if you doubt, then their period is three months, and [also for] those who have not menstruated. And for those who are pregnant, their term is until they give birth. ...