2nd edition decanter of endless water, worth 1,000 xp
Stream: pours out 1 gallon per round
Fountain: 5-foot long stream at 5 gallons per round
Geyser: 20-foot long stream at 30 gallons per round
3.x edition decanter of endless water, worth 9,000 gp
“Stream” pours out 1 gallon per round.
“Fountain” produces a 5-foot-long stream at 5 gallons per round.
“Geyser” produces a 20-foot-long, 1-foot-wide stream at 30 gallons per round.
7.48051948 gallons in a cubic foot. So on Geyser it fills roughly 4 cubic feet per round. A 10'x10'x10' room is 1000 cubic feet, so it would take 33 rounds to fill.
Now, dungeons tend to be fairly big. I have no idea how big. Let's take a rollable battle mat, say that a common 24" x 36" mat is one floor of a dungeon, that is a 1" to 1' ratio, and that perhaps 1/2 of the map is actual open space. 24x36=864 square feet x 10 = 8,640 cubic feet / 2 = 4,320 cubic feet per level or 1077 rounds to fill a level, which is less than two hours to fill a dungeon level. Yup, decanters fill a dungeon level fairly swiftly (when on Geyser -- if left on stream then it's 30 times longer or 2 1/2 days to fill a dungeon level.
Now, how about to fill a valley? I'm going to use Lake Arrowhead in California for that because I grew up there, and also because it's a man-made lake. Anyway, the lake is a decent sized lake and fills one valley. It's nothing like Lake Powell, though, it's much smaller. One acre foot equals 43,560 cubic feet or 435.6 hundred cubic feet and is equivalent to 325,851 gallons. Lake Arrowhead has a surface area of approximately 780 acres (320 ha) and a capacity of 48,000 acre·ft.
That's 48000 acre feet * 325851 gallons in an acre foot / 30 gallons per round / 10 rounds in a minute / 60 minutes in an hour / 24 hours in a day / 365 days per year = 100 years to fill Lake Arrowhead from a decanter of endless water.
Although I could see a decanter being a serious problem in a dungeon, especially since the lower more valuable levels would fill up before the higher levels, and since even 2' of water in dungeon level starts becoming seriously annoying and anything over head height starts to be seriously life threatening (and some races aren't that tall), I don't see a decanter being a real problem outside.
Let's consider just Papoose Lake, which you can see at
https://www.google.com/maps/pl...+Arrowhead,+CA+92321 next to Lake Arrowhead. 35 acre feet * 210 feet deep is 35*210*325851/30/10/60/24/365 15 years to fill although that brings up an interesting question, how much not to fill a valley and create a big lake, how much just to make life difficult for halflings? And let's use a titchy small thing like Papoose Lake. Seriously, I can run from one end to the other in just a few minutes, it's not very big. 35*2*325851/30/10/60/24 = 52 days to make the water 2' deep. By then, someone out to have noticed the gradually occurring flood. The deeper the water gets, the more slowly it's going to take to rise since the ground likely slopes and every foot of rise will mean even more acre feet to be filled.
Tl;dr A decanter of endless water is likely a huge problem in a dungeon, but isn't really a problem at all outside. Outside is freaking huge.