Written with people moving from Turkey in mind. The Portuguese rules still apply more broadly.
Fines for Overstaying Visas
If you stay in Portugal past your visa expiry without a pending AIMA appointment, you are illegal. If caught exiting the country, fines range from €80 to €700 depending on the duration of the overstay.
More importantly, you may be flagged in the Schengen Information System (SIS), which could lead to entry bans for future trips to any EU country. Always carry proof of your AIMA appointment to justify your stay.