BareBear in mind that this will veryvary by country/institution etc. according to rules and time limits.
Reasons for exceeding 2 years could include:
The student worked slowly/poor time management
The student decided to extend the scope of their thesis (as part of a larger project or to include an internship/secondment)
The student decided to change thesis topic
The student's project involved an experiment which caused a delay
The student was waiting for an available time to defend their work (if an oral defence is a requirement) or other administrative reasons
Any of the above (or a combination) could be the reason for a thesis taking longer than a year to complete. If you have concerns I'd suggest contacting a current student(s) or students in the working group you're hoping to join and asking a few questions.