Assuming you first sanitize the tank using this procedure, add a small amount of bleach to each fill and you should be good for a few weeks. But, of course, it depends on the quality of your water and whether you plan to drink it or use it merely for washing.
Depends. If it is unchlorinated well water, I would not leave it over a few days. Chlorinated city water, maybe 5 days tops. However, for an emergency supply you can fill the tank to the brim and stabilize it with pure chlorine bleach at the rate of 1/4 teaspoon per gallon. That should be changed every 6 months.
You folks are really conservative! We have kept water for several weeks without problems, though I would normally drain and refill sooner than that if I had the opportunity. Continually adding fresh chlorinated water also helps, by restoring the chlone level in the tank somewhat.
Chlorinated water remains safe for use quite awhile, but the amount of time for any individual case is not treally predictable. As has been stated, it depends on the conditionof the tank, the initial quality of the water, amount of chlorine present initially, and what it is exposed to after filling. And there may be lots of things in the water that make you squeamish but really are not health hazards at all. Most algae is in that category.
And some people are more sensitive to micro-plants and microbes in the water than others. If your own "plumbing" is not routinely exposed to the various algaes and microbes that are common in many areas, you may have an unpleasant reaction. The infamous Mexican "Montezuma's revenge" is an example - the Mexican natives aren't bothered at all but visiting gringos may get the trots or other symptoms real quick.
Yeah, I am conservative. Back in the old days of sailing ships, they carried their water supplies in wooden barrels for months in the tropics. It was famous for algal blooms and critters. However, one should remember that the jolly tars were young, vigorous men who had survived a youth in ports with 18th century sanitation stations. They had the immune system of a turkey buzzard. They also drank their water largely in the form of grog -- 3 parts to 1 part 140 proof rum and a part of lime juice (the original daquiri). Hell, with that mixture you can drink raw sewage.
In our case, however it is easy to be conservative in campgrounds with sources of plumbed sanitized water readily available. Drinking ancient water is no longer necessary. And with our delicate and often ancient immune systems is worth caution.
However, if in doubt, the dilution of 2 parts of water with one part of a good bourbon or scotch may be a wise precaution in the evening.
By the way, Mexicans can get tourista. The Valle de Mexico (Mexico City and environs) has an endemic bug, I forget the name, that causes serious diarreha. Mexicans native to areas outside if it can fall prey to the bug. Mexican urban watersystems are less than isolated from the local soil and its critters. Mexicans who can afford to either drink bottled water, or have special domestic purification systems in their homes. The other folks get a lot of waterborne disease and gain 18th century immunity -- the hard way. They also die early, like the 18th century folks.