Deligne's Weil I has been published under the title "La conjecture de Weil: I" in 1974, and Weil II in 1980. So did Deligne know in 1974 that there would be a Weil II, and can one explain the period between the two publications?
To complete Carlo's answer, I think that one thing that can explain the long gap (in addition of the amount of difficult material in Weil II) is that Deligne felt the need to consolidate his result of Weil I before going further.
It should be reminded that Weil I was criticized from various directions for relying on results that were not yet formally published. Those results were essentially the theory of Grothendieck of application of Etale Cohomology to L-functions (SGA 5) and the theory of Lefschetz as generalized to schemes by Grothendieck (SGA 7). At the time of the publishing of Weil I, Grothendieck had left the IHES and was not working anymore on the SGA's (nor the EGA's), and his students and colleagues were left with notes of his talks in various states of redaction. Serre tells, for example, in a letter to Grothendieck (published by the SMF in the volume "correspondence Serre-Grothendieck"), that Illusie, who was in charge to prepare for publication some crucial parts of SGA 5, told him he was not able to check the commutativity of certain diagrams, commutativity considered as obvious in the notes.
So something that occupied Deligne for quite a while between 1974 and 1980 was this huge work of publishing/completing the work that Grothendieck left abruptly in 1970 (how much this work was just cleaning, proofreading and publishing, and how much it involved original mathematical work is the subject of polemics in Grothendieck's "Récoltes et Semailles"). For instance, Deligne (with others) published SGA 4+1/2 in 1977 with the stated intent to be a partial substitute for the then missing SGA 5. Deligne also worked with Katz and the second part of SGA 7, etc.
This is what Pierre Deligne writes himself in the introduction to part I about the need for a part II:
I would think it took 6 years to complete part II because the Hard Lefschetz Theorem is, well, hard.