Assumption: We are talking about Green Hair Algae, right?
How long before it starts to disapate? That's a much harder question. That partially depends upon nutrient availability. Based on my experience, I would say anywhere from 2-6 weeks. Maybe longer if you have two much of a surplus of nutrients in your tank.
There are a number of things you can do to help the situation along. I will mention those in a moment. However, first off, I recommend doing nothing extreme or terribly quickly. Only bad things happen fast in this hobby. Don't pull out all your rocks and scrub them off or anything. Just take your time with it.
Ok. Some things you can do.
1. If your tank is low on herbivores, add some to the tank. Not a lot, I'd like the giant turbo snails that are sold all over the place. Perhaps a snail or 2 per 50G of tank. More might go through the algae faster, but will only die later.
2. Check your pH. According to my reading, GHA can be inhibited by maintaining a pH of 8.2 - 8.4. If your pH is low, the environment is more ripe for the growth of GHA.
3. Do you have a protein skimmer? If not, blow some money on an
AquaC Urchin or EV-120. And then make sure that you have a decent amount of flow in the tank. Aim for 20x GPH flow minimum. And remember here that laminar flow isn't what you are looking for. So we are talking about added some SEIO or Tunze Stream powerheads.
4. Consider adding a Remote DSB (RDSB). You can find plenty of info on these in Anthony Calfo's old forum here, All Things Salty. Or head over to his new forum and check for threads there. if you need a link over there, PM me.
5. If you don't have some, get some Chaetomorpha (Chaeto) from someone and toss it in your sump (refugium?). Get a good daylight floodlight bulb from HD/Lowes/the web and put the sump on a reverse daylight cycle. That will help keep your pH up as well.
That's it for my GHA advice. Take it all with a grain of salt (pun intended).
One more thought. Let your Calcium drop to 450. There is nothing that you're going to put in your tank that is going to need that high of a concentration. I'd even sat 420 is ok.
Whatever the case, good luck! I hope things go well with your tank!!!
