Or looking at the law:
https://diariodarepublica.pt/dr/legislacao-consolidada/lei/1981-34536975
Residirem legalmente no território português há pelo menos cinco anos
Wiktionary says this is residirem - Wiktionary, the free dictionary
third-person plural future subjunctive
Which is above my Portuguese language knowledge. I guess you have to be a lawyer.
@jb4422 anyway to your point, one concession the US makes is that you can apply 3 months before your actually 5 year mark, presumably that was put in the law when processing times were around 3 months. Similarly Portugal could change the law to say, e.g. if processing times take 2 years, you can apply 2 years before your 5 year mark, the only restriction being they can’t complete the processing until you reach 5 years.
Which, in a way they’ve done by moving the start date earlier!