When are we talking about? 1960s-70s? 1980s-90s? 10 years ago? Big difference. Brown Mexican seeded? Columbian gold, Panama red, Skunk, Hawaiian, Thai, Afghani....
Once Indica started making its way in to US weed, hybrids, the kind of studf they were growing in California 40+ years ago, quality was already going way up. Over the last 30 years we're talking incremental increases in thc% through selective breeding, the development of new varieties......
The answer to how long it takes for thc to form on a plant........and of course it really forms on the outside of the various plant parts, in the resin glands, for the most part on an increasing basis over the life of the plant......there's a small amount of resin on fan leaves even early in a plant's life. Unless it's some amazing genetics, you'd be hard pressed to see much resin, even with magnification. You'd have to smoke quite a bit, it would be pretty harsh, and even with low tolerance you'd probably get a mild buzz, maybe with a sore throat. As the plant grows and matures, more resin forms on the newer growth, the smaller leaves, shoots, tops......still not very much. Ultimately it's going to be the female flowers where the real resin production is, otherwise there would be a decent market for males. Even with the early female flowers resin production is relatively minimal, but still noticeably more than the leaves. As female flowers/buds start to really develop and mature, that's where resin content goes up considerably. Now you can start seeing the many resing glands glisten and sparkle in the light........
2 days? That's not really going to decrease your tolerance.........try 20 days. ;). Might as well smoke a bud that's broken off, but an early, small flower site is going to produce a heck of a lot more than a few hits after many more weeks. I know it's hard to wait, especially if you don't have any weed while you're growing, but the more patience/willpower you have, the more you'll be rewarded at maturity.
At each stage, every few days or so, depending on how many plants you have, how big they are, you can pinch off a little something, but try to properly dry it and even cure it a bit.......you'll see that each bit you harvest will be stronger than the one before.
Another thing you can do is a "water cure". Thoroughly dry whatever you take, even the leaves. Then soak it in cold water for 12-24 hours. Leave it as ibtact as you can so the resin stays stuck on the material. The cold water soak will pull out the chlorophyll and other unwanted plant matter and you'll pour off this greenish water.......re-dry the material and it will be stonger and less green/nasty tasting. Use a bong to further clean the smoke.