We can use 'will have done' to talk about what will have been achieved by a certain moment in time.
- We'll have been in these offices for eight years next month.
- She'll have visited ten countries in twelve days by the time she gets back.
- I'll have finished this project by Friday.
If we want to emphasise the continuity of the activity, we can use the continuous form.
- I'll have been working here for 35 years by the time I retire.
- She'll have been driving for more than fifteen hours straight by the time she gets here.
- They'll have been working with us for 15 years by the end of this year.
We can also use 'will have done' to predict what we think has already happened at present.
- He'll have already read the report by now. Too late to change it.
- She'll have boarded her plane. It's too late to contact her.
- They'll have decided by now. We should hear the result today or tomorrow.