How long can a pitcher of tap water be left out, and still be drinkable?
If you have a clean glass pitcher, the length of time we're talking is months, as mentioned by Tom's answer.

I think you have a few questions that I can clear up:


The bubbles that form over time are dissolved gases. From the faucet (or pitcher), the act of pouring will force some air into the water. Over time, it will warm up and you'll see bubbles form along the glass. This is absolutely harmless.
Flies laying eggs? Well, I wouldn't leave a glass pitcher out and uncovered, but I highly doubt this. Flies want to lay eggs in places where the hatching egg can live. For example, trash, spoiled fruit, the gunk in your drain, etc..
Chemical reactions with dissolved ions? Since most water in the US is fluoridated, there will be some very slow etching of the glass by fluoride ions. The amount of this will be very, very small, since it's a slow process. I think you'll easily be safe over the timescale of days to a week or two.


So what happens to water left at room temperature over a few days? Mainly the dissolved gasses come out. I find this water to taste "flat" even if I cool it again. But there's not much that will happen to the water.

More than a few days and I'd suggest making sure the water is capped or closed to avoid dust or other things falling into the water. Maybe find a closable glass container?