I have a cheap Stanley 24-inch level like this one:
The other day I noticed it didn't seem to be very accurate. I checked by flipping it end for end on my workpiece, and sure enough the bubble settled in a different spot.
To measure how far off it is, I propped up the two ends with stacks of about 15 sheets of printer paper on my kitchen counter. (It was important to support both ends because my counter isn't perfectly flat). I added paper to one side until the bubble was dead center, then flipped the level end for end and had 9 sheets of paper (0.036") to one side before it was dead center again.
atan(0.036 / 24) = 0.086 degrees. Since the vial is crooked and I'm flipping end for end, 0.086 degrees represents the angle between it being crooked to one side and being crooked to the other side, which is double the crookedness. So the vial in my level is crooked by 0.043 degrees, +- 1/64" over 2 feet, or +-0.00075 inches per inch.
That doesn't sound like much, but it means that if I set two 8 foot beams indicated as "level", but flip the level around in between, they'll vary by 1/32 every 2 feet, for a total of 1/8" over the whole length.
Stanley doesn't make any accuracy claims for my level, but their fancier "professional I-beam level" claims accuracy of only 0.0015 inches per inch, twice as bad as mine! But then their 24-inch aluminum box-beam level claims 0.0005 inches per inch, somewhat better than my 0.00075.
A fancier brand, Stabila, claims 1/32" over 72", which is also 0.0005 inches per inch. The manual for their fancy digital level (which doesn't exactly inspire confidence with its poor formatting) claims 0.05 degrees within 0.1 degrees of level, and 0.2 degrees elsewhere. So that's worse than my crappy level's 0.043 degrees even in the best case. But then just below it they have a heading of "measuring accuracy of level" and claim 0.029 degrees, which is pretty confusing but just works out to the familiar 0.005 inches per inch. Bottom line: the level could be off by 3x the allowed 0.0005 inches per inch, and the digital readout could still read it as dead level.
So the bottom line seems to be that for best results, I want a spirit level that'll allow me to adjust it, and even fancy digital levels aren't guaranteed to be any better than perhaps 3/16" over 8 feet.
(Just for my future reference, I also measured how much angle change there is between level and when the bubble just touches the first line. That came out to 0.120" (about 1/8"), so a rise of about 1/16" per foot.)