The water should be clear and blue. Look for any cloudiness in the water. Make sure you can see all the way to the bottom of the pool, even the deep end. If the water is tinted green or it is cloudy, it may be contaminated with algae.
I think the answer to your question is about 3-6 days. The problem is that the chlorine that you need to keep the bacteria in check is used up more quickly as the temperature rises, the activity increases, and as sweat and other body stuff is put into the pool.
The most commonly reported recreational water illness is diarrhea — often caused by germs such as E. coli, norovirus and Cryptosporidium (also referred to as Crypto). When a person with a diarrheal illness enters the water, the entire pool can become contaminated.
But is it safe to swim in a pool with algae? Whether mild or severe, it isn't recommended. Significant amounts of swimming pool algae welcome a breeding ground of harmful bacteria that feed on algae. These bacteria pose health risks to swimmers, most commonly resulting in a skin rash.
What's important for you to know: You shouldn't swim in a pool that's green until you test the chemical levels, like the officials in Rio did before the diving event began. It's the balance of things like chlorine, pH, and alkalinity in a pool that keeps it sterilized.
Test the pool water for bacteria, yourself
The Watersafe Bacteria Test is a rapid test kit that detects the presence of bacteria in swimming pools and spas.
In most cases, the walls and floor of your pool feel slimy and slippery due to a lack of pool maintenance. Water chemistry is important in order to maintain clear and bacteria-free water. Generally, slippery and slimy pool walls are an early indication of bacteria and algae growth.
Brushing thoroughly 1-2 times per week will prevent these materials from having the time to create a stain. Brush to Prevent Algae: Every pool can grow algae, but plaster, quartz, and aggregate finishes are more susceptible.
Pool water turns green because of algae in the water. Algae can grow rapidly, particularly when it's warm like Summer, which is why it can surprise you overnight. This generally comes down to an imbalance or lack of chlorine in the water.
The most common form of algae in swimming pools is "green" algae. Green algae varies in color from blue-green to yellow-green to dark-green. It can be free floating in the water (turning the water a hazy green) or can cling to the wall-clinging (patches of green).
Green Algae
It can grow both on the pool surface, resembling a green, slimy stain, and also suspended in the pool water, causing the water to appear to have a green tint. In severe cases, the algae is concentrated enough so that it is impossible to see the pool bottom, or even the steps.
Proper chemical balance and sanitizer levels will prevent many opportunities for algae to bloom. High pH and low chlorine (or other sanitizer) can give algae a great start.
It takes a lot of chemicals to make pool water safe for swimming. Untreated water can accumulate harmful Escherichia coli and Salmonella bacteria and protozoans such as Cryptosporidium parvum and Giardia lamblia.
The itchy rash associated with swimmer's itch looks like reddish pimples or blisters. It may appear within minutes or days after swimming or wading in infested water. Swimmer's itch usually affects only exposed skin — skin not covered by swimsuits, wet suits or waders.
Make sure to rinse your hair and wash your swimsuit after you swim. Showering after you swim will keep your skin from becoming dry. You don't have to worry about picking up a germ, and you'll get rid of that chlorine smell. It only takes a few minutes to shower, so make sure you make it a priority.
Remember: vacuuming your pool regularly will help keep your pH balanced easier and more controlled. Skim your pool for debris, such as leaves, bugs, etc. and be sure to check your skimmer and empty that regularly as well.
Brushing the walls and floor of your pool will indeed prevent algae buildup because it prevents those algae spores from attaching to the surface and beginning to grow in the first place. It is always good to remember with algae that it is better to prevent it than to treat it.