The seminars are usually about one hour and 30 minutes long but the time varies so that the seminar could last for the whole day as well, in which case all the breaks needed are usually taken.
If no further information about the time, the seminar starts a quarter past the even hour (e.g. 12 pm). If the seminar is meant to begin e.g. 12 sharp or 12.00 it means usually that it begins on the hour not a quarter past.
The attendance is usually always checked, whereas in lectures they are not always checked nor are they mandatory as the seminars many times are. Some lecturers might circle a paper to list the participants, but not all, and if the lecture is not obligatory, no attendance is usually taken.