You are right, the log tables do update frequently - they are a record of every action that takes place. As such, as Teresa says, they are probably the best place to get the information you are looking for.
I'm not really sure of your point though - if you have a live moodle site, then teachers are going to be constantly interacting with it, hence the frequently updating logs. You have asked how you can get the information about how much teachers do with the site, so you are innevitably going to be working with a constantly changing data-set such as the log tables in the database.
The only way around that really, is that if you want to present the information as at a particular time, as a snapshot, then take a backup of your database (or that particular table) and work with getting the information from the backup/exported copy rather than the active live version.
Richard