It will possibly be done in id say three to four weeks, now is when it would start bulking up, but it looks like you ran into some issues in this last stretch, i'm not sure you mentioned or i missed it but would you care to elaborate? Missed watering, plant ran too dry, or a deficiency which caused a self cannibalism of old leaves... ?
Also how far away is the light at the moment ?
As for flushing if you "want to" give it another week and half plus before you think about it.
As for the COB's the cheap ebay ones are pretty useless in comparison to a well made out one with quality parts, so much so you'd be better off running commercial screw in led bulbs with the covers popped off, only down side is they take up a bit more height than a Homemade LED COB light, but are perfectly easy to find and setup and are cheap alternatives for the amount of light you get.
Say if i made a new cab that was 1sqft i could run 4 - 14w Led bulbs which would be plenty of light, id be looking at $7 a bulb plus fixture of say $2 a piece, so round it out to say $40 total unless the bulbs are on sale, where as a good cob design your looking at say $10 a piece for 4 LED's, 2 drivers at another $10-15 a piece, and heatsinks for $5-10 a piece, so $80-110 not including shipping, sure you get more light, but its harder to upgrade or fix issues aka burnt out leds which can happen. Really all depends on what you want to do
, i should note that more sources of light has worked way better for me that just one singular bright source.
I'm thinking about going screw ins for my next build, i went with cob's this last time because of my height clearance, and to just try something new, but now that I've been there done that, i realize sometimes the easier things have a bit more weight to them, than ultimate efficiency, as for reference i use to grow with cfl's and floro tubes in the past, and would jump to screw in led's if height wasn't my issue.
Also Quick Recommendation: Remove the dead leaf clutter, it can create mold and attract bad bugs and pathogens, you will run into less issues if you keep it clean.