How to Use the Future Perfect Tense1: We use the future perfect to say 'how long' for an action that starts before and continues up to another action or time in the future. Usually we need 'for'. We can also use the future perfect continuous here so we often use the future perfect simple with stative verbs. If we use 'when', we usually need the present simple.
- When we get married, I'll have known Robert for four years.
- At 4 o'clock, I'll have been in this office for 24 hours.
- I've lived here for 11 months and three weeks. (This is correct, but the time is not an easy number.)
- On Tuesday, I will have lived here for one year. (A much easier number.)
- By 10 o'clock, I will have finished my homework. (= I will finish my homework some time before 10, but we don't know exactly when.)
- By the time I'm sixty, I will have retired. (= I will retire sometime before I'm sixty. Maybe when I'm fifty-nine, maybe when I'm fifty-two.)
If you want to learn more about the tenses, especially about how to use them when you're speaking, you could try Perfect English Grammar Plus.