In this case, the other person is predicted (expected) to arrive 3 hours in the future:

Either way, the Reference Point (R) remains connected to the nominated future time period (by the time you get here). Note that by using the Future Perfect (Continuous) form, the timeframes are very clearly expressed. For this reason, the two clauses can be formed in either order and the meaning is maintained:

• I will have been working for four hours, but the time you get here.
• By the time you get here, I will have been working for four hours.

A note to remember is that not all verbs are used in the continuous form. If a verb described a permanent (semi-permanent) state of being, rather than a dynamic activity, it is common to use a simple verb form, rather than a continuous verb form. This applies to examples such as "are married" (not "we are being married"). In this case, we would say: By 2038, we will have been married for 20 years. This is because the action of being married is ongoing, and is expected to continue. Likewise, you could say, By 2020, I will have been finished college for two years. This is correct because the status (or state) of having finished college still stands. You can read more about this in Talking About Feelings.

For more on the Future Perfect Continuous, see our section on Combining Actions.