Two things:

1) The "Online User" text is poor wording since that part of MVNForum is reporting on cached data. I've updated the text for that so that it says "Recently Active User". This is a more accurate description. Because of the caching you can be "Recently Active" and signed out. Please don't use that section to determine your logged in state. Only the presence or absence of your user name in the top left of the screen can accurately show if you are logged in or not.

2) Your login state on the website is determined by the LTPA cookie. When originally issued, the cookie will allow you to stay logged in for 6 hours. The server would cache the value of that cookie for 4 hours. If you logged out and logged in again within those 4 hours, you be issued the same value for your LTPA cookie (and thus have the same expiration time as when you originally signed in). A couple of examples to clarify this:

Example 1
Login at 9:00
You will be issued a LTPA cookie that will expire at 15:00
Logout at 9:30
Login at 12:30 - you will be issued a LTPA cookie that matches the first and therefore will expire at 15:00

Example 2
Login at 9:00
You will be issued a LTPA cookie that will expire at 15:00
Logout at 9:30
Login at 13:15 - you will be issued a LTPA cookie with a new value that expires at 19:15

Note that the LTPA expiration time assigned is a fixed time. It does not change if you continue to access the website during that time.

In order to simplify this and to make things behave more consistently, we have extended the LTPA expiration token time to 8 hours and reduce the cache to 10 minutes (the only reason for a longer cache is performance but the performance of this function is fine so we can easily handle more cache misses).