daf bit: Nazir 7

If a man vows to become a nazarite but does not specify an amount of time, the vow is in effect for thirty days. If he specifies a time it is in effect that long (so long as it is at least thirty days). He can specify time in terms of "for as long as it takes to go from here to (some location)". What if he specifies "for as long as it takes to go from here to the end of the earth"? This means thirty days, not for the rest of his life. (5b and 7a)

Earlier in the gemara we get, in passing, that this kind of vow is considered a sin, at least some of the time. (There's probably more on this.) Yet we have an entire tractate dedicated to the subject. Hmm.