First, the MINIMUM time the average beer takes to fully carbonate is 3 weeks at 70°. That's the minimum, many beers will take longer. The higher the abv, the longer they will take to fully carbonate. Big beers can take months. And at colder temps it will take considerably longer. Even one day at cold temps can drop enough yeast out of suspension to cause the carbonation to take much much longer.
Second, to accurately measure your priming sugar, you really should be doing it by weight. The volume of 4oz of sugar will vary considerably depending on how fine grained it is, and how tightly packed it is.
Third, 1/4 cup of corn sugar will likely weigh somewhere in the neighborhood of 1.8 oz, certainly under 2 oz. Assuming a 5 gal batch size, this is only going to result in ~1.5 vol of carbonation. That's cask ale levels, and most people would consider that very flat. There's nothing wrong with lower carb levels, and a lot of people prefer lower levels for stouts, porters, and a lot of English ales, but if you want something similar to what you'd find in the vast majority of commercial beers, you should be shooting for 2.4-2.7 vol. Even your batches with 1/2 cup are likely only going to end up at around 2.2 vol, which is pretty light carbonation.
There are numerous calculators online that will tell you how much sugar you need to reach a specific carbonation level. They'll even adjust for different types of priming sugars. Here's one-