I just realized you might be asking how to store info for non-logged in users.
In that case I would do the same thing as above, but use a hashed version of the client context data as a unique user key to keep my timestamps.
Then I would definitely be using a separate table from users, but still storing the login timestamps in a simple object column, and a unique key column that would either be the logged in user or this hashed context info to avoid duplicates.