When can I harvest?

4-5 more weeks. when the calyxes plump and make the stigmata look short, then you are maybe 10-14 days away. Even then ripening takes a bit longer. Definitely get that 30-60x magnifying glass of some sort - currency checker or jeweler's loupe etc. Get glass lense and read the comments. Many of them lie about magnification. you'll find more than a few marijuana growers in the comments on amazon to help out and possibly on other online sites too. you don't need to flush. You just need to avoid overfeeding. if the substrate ever becomes toxic, that's when you "flush" the medium to dilute it. If it is at a safe concentration level, then all is fine. Consider outdoor growing. The soil always has nutrients, do they flush outdoor grows? No, because it'd be absurd. But, if you have your heart set on flushing it won't do anythign too negative. Just wait for the buds to actually look plump and bud-like. Then you have 7-14 days, probably, depending on what you deem looks like a finished bud. Might have some bias in ya, based on the 2-3 week guesstimate here. Don't try to pick the timeframe. let the plant tell you.

You'll have time to get a loupe. At least 3 weeks

You have an other 4 weeks to go.