A season is approximately three months. "Weeks" is pretty clearly a significant portion of a season, and if "rust" refers to a grain blight, all the more reason to think in seasonal terms since that's literally about growing seasons and a harvest failure that won't manifest until...well, harvest time."Rust" is a mold that infects wheat, so I'd say more on the order of weeks than seasons, but okay. I'm not saying the character shouldn't have immediate offensive capability. However, the warlock is designed to focus on that. If your warlock character wants to take the subtle approach, they can, sure. However, a bunch of invocations are designed to focus on eldritch blast; if you choose to focus your warlock on other abilities, there is less mechanical aid.
D&D's rules just don't support effects on the timeline of multiple weeks or longer, and a class built exclusively around such effects would be extremely difficult if not impossible to play in the vast majority of content. A class not built exclusively around that...would get features approximately equivalent in impact to things like eldritch blast.In any case, you can certainly play your character any way you wish, but telling someone, "Your character doesn't really seem like a player character," is a little bit of a judgement.
That is why I said what I said. You are either proposing a character class that lives and works at a time scale that doesn't fit into the mechanical structure of D&D (and probably never has, even back into the logistics-and-heisting old school style), or you are proposing something that the Warlock already does reasonably well, especially if you choose to play Pact of the Tome and take Book of Ancient Secrets. Indeed, the BoAS Tomelock is uniquely suited to being an excellent ritualist because, unlike any other class in 5e, it doesn't have a limit to its ritual learning: ALL rituals can be scribed into the Book of Shadows, regardless of class, so long as you can find a copy.
A class built exclusively around effects that take even weeks to truly manifest is simply not something that D&D is designed for, and one that is not built exclusively for that will already find a home among the extant classes. Doubly so if you go Tomelock and focus all of your Invocations on utility effects rather than combat; you don't even have to take eldritch blast if you don't want to, and Tome gives you three cantrips from any list you like. Take, for example, shillelagh or primal savagery (melee option), message, and druidcraft to create a classical "witchy" type character, or sacred flame, guidance, and spare the dying to be a medicine-man type healer-seer (with mage hand and your choice of other baseline Warlock cantrip, probably prestidigitation.)
Invocation choices could be something like (in order taken): Book of Ancient Secrets and Beguiling Influence, One With Shadows (upgrading to Shroud of Shadow at 15), Devil's Sight, Whispers of the Grave, Ascendant Step, Master of Myriad Forms, Visions of Distant Realms. This produces a character replete with "subtle" effects, powerful at-will magic (especially once you hit level 15), and an emphasis on social and investigative things rather than offense.