I am guessing 10 weeks or so.
It'll probably go away on it own eventually. When I pluck out a sprouting of the stuff, it doesn't seem to be growing back in the same place. I bet if you harvested it that it would help make it disappear. Handy harvesting tip: when siphoning out water during a water change, use a 1/4"-3/8" i.d. vinyl hose, and hold it underwater in the tank near its end so you can use a finger to cap the hose off. Allow the algae strands to get sucked into the hose down to their base, then tightly cap the hose with your finger, pinching the strands in the hose, and pull to pluck them free from the rock. Then uncap the hose and the strands get siphoned down into the bucket. Works pretty good for hair algae too. It doesn't get it down to the rock surface like scrubbing would, but I've seen hermits go after fresh trimmed hair algae that they wouldn't touch beforehand, and they'll then clean it down to the rock. At least in my case.
