Hi:
If you often log into and log out of the forum.
You visit the forum very often.
But, you can also log in and out many times in a minute.
So, "log in" or "log out" does not mean you stay here for a long time.
If you say "spend a lot of time here", perhaps you can log in in the morning , and spend a whole day learning English here before you log out at night.
Another case where you can say "spend a lot of time here" is when you have been visited once every a few days in the past 5 years.
A lot of /a great deal of time means the length of time is long.
"often" means "frequency", "how often".
You're free to ask if you don't understand.
Best wishes