You are asking about the future perfect tense: will + have + verb past participle.
This is used for a future action that will be completed before a specified time in the future.
1. By the time I graduate, I will have saved enough to ...
2. By the end of this term, I will have saved enough to ...
Would you please tell me the rule when to use the simple form and when to use the future form after by the (time adverbial), since the use of 'will' is not permitted in time clauses.
'By the time I graduate' is correct.