Alright, that makes sense. I usually do route marketing some 1 months, maybe 2 at the opening of a new route. Especially for promising ones and with competition. Thing is as I'm growing all the time route marketing never slows down. So it's a big part of my costs actually.
Ditch it. Altogether.
It's a broken game mechanic and it doesn't work. Hasn't ever worked. All it does is separate inexperienced players from their cash while experienced players who know better save that money and use it to run the newer ones out of business.
be able to make an argument on the first couple routes you ever operate because the costs are somewhat low, but even then it's literally only shaving a few game-days off the time it takes to get to an RI of 100. If you've got competition that's saving that money for an extra plane that doubles their revenues while you're trying to eek out an extra couple percent a couple game-days earlier, you're gonna lose.
Try it. Open 2 routes at once, spend the money on one of them, don't spend it on the other. And then watch how the RI grows. You'll barely notice any difference at all.
As for after your first couple of routes it's insanely expensive. It's broken and grows linearly with the size of your airline. So your 100th route costs many, many
times per week to market than your first one. Pretty soon you'll be spending more than $1M/wk just to route market on some tiny route that won't generate that much profit in a year even once it gets to 100 which, again, it will get to anyways. A gigantic waste of cash even if it did
provide some benefit.