Are there any laws (on any level, i.e. EU or Spain) that regulate how long one can stay at a place before special measures
have to be taken ..?
After 90 Days you need to register and apply for a NIE
From 2007 Spain requires (Royal Decree 240/07) that all EU citizens planning to reside in Spain for more than 3 months should register in person at the Oficina de Extranjeros in their province of residence or at designated Police stations. You will be issued an A4 printed Residence Certificate stating your name, address, nationality, NIE number (Número de Identificación Extranjeros) and date of registration.
You will also be issued a card that is acts like an ID card . Valid Max 5 years
A small note : Since European members can not decree / legislate / regulate issues in conflict with the EU law , it is safe to assume that these regulations exists on some European level or another. I can find the source but I do not think it is really the issue.
Why do you need it ?
Well , as many argue you don't - you will need this card in order to complete other procedures , like opening a bank account, opening a Business, applying for a job, legally buy a SIM card plan, or even apply for visas at other embassies without the need to return to your home country .
It is true that almost all those things can be also achieved without the card - ( evidenced by the many
sin-papeles living there ) It is much easier with.
Is it only a burocratic procedure?
Yes and no. Since 2012 Spain passed some decree that allows (not require) local officials to ask for proof of sufficient financial means of support, and also proof of public healthcare insurance. Most likely, being a EU citizen, you will not be asked for one. But just know you might .
Legal Implications ?
Legal trouble - Most likely NOT.
The EU Citizens has a
freedom of movement (Note :
Movement. Not residency)
Since there are no border controls - No one can really know when you arrived or how long you stayed . So Legal issues are not likely to occur .
BTW - regarding this freedom of movement - It is not inclusive for all states - there are some phases (or were) of the implementation of movement - but it applied to non original members and abolished in 2011 (only Germany and your home country Austria actually applied it as far as 2011).
Note that A person can have only ONE residency (as opposed to multiple citizenships) - So effectively - Your Spanish residency will expire your Austrian one . That is not a real problem if you really live there. You can also always reverse that process with ease .
I don't have any definite plans about how long my trip will last in
total as well as how long I will stay at each of my destinations
individually I suggest you wait a bit before applying .
My suggestion - See if things go well for you there - and only after you will see that you really want to stay - apply for the ID and go through the process.
Don't rush into it before you are sure you stay.
On a personal note - Spain is a really great country to live in. You will have a great time!