Without hunting back for ages, I'm not sure how long an investigation can stay open if there are no arguidos. Offhand, i'm not even sure that it's specified.
Once there are arguidos, the ball game changes and the clock starts ticking.
That can put quite a stress on both the investigation team and the arguidos themselves. I haven't quite understood the reasoning as to why they have to retain that status (imposed or requested) until a final decision is made.
https://www.ft.com/content/ca71ca96-ae89-11e7-aab9-abaa44b1e130Former Portuguese PM Jose Sócrates charged with corruption
Peter Wise in Lisbon OCTOBER 11, 2017
snipped
........ The formal charges come almost
three years after Mr Sócrates was named as an arguido, or formal suspect, in the case. ...................
After being named a suspect in the case in November 2014, Mr Sócrates was held in jail for nine months under laws that enable investigators to detain suspects in serious cases for long periods without bringing formal charges while they pursue their inquiries. He was released from detention two years ago, but formally charged on Wednesday.
The former prime minister has repeatedly accused state prosecutors of abusing his democratic rights by failing to bring formal charges sooner.
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It seems there are exceptions to the rules.