odds in calling river bluffs

In many hand histories posted on this site, where hero has a good but non-nut hand and is facing a reasonably large river bet (fold/call decision), a common claim is that you only have to be 'right' 1 in 6 times (or sometimes it's stated as 1 in 7 times) to make a profit. Is anyone familiar with this, and if so, can I get further explanation? I've got to think that my understanding of this claim is way off - how can calling and losing with 2nd best hand 5 times in a row be justified if the sixth time you catch a bluff and win? I realize that dead money in the pot often makes the pot odds for calling a river bet with a potentially winning hand pretty good, but where does the actual ratio of 1/6 or 1/7 come from? This seems quite arbitrary - Is it dependent on the situation, i.e size of river bet and/or size of pot, and if so, what is the equation to figure out the profit ratio for a given hand?

I should have an example or link but I don't - I apologize if this post is complete nonsense.

someone else can certainly give a better explanation, but this 1/6 thing relates strictly to pot odds. For example, in Limit game if there are 15BB's in the pot and you have missed your flush draw but picked up a pair on the river (like paired your 7 or something) - if it costs you only 1 more bet, you are being offered around 16:1 on your money - so in this case, if you call 6 times and win once, you will gain BB's overall - The same thing is true in NL - If the pot is $1000, but only $100 is bet on the end, you can reasonably call with hands that you don't think are the winner, strictly because of the odds you are being offered...im sure there are more exact calculations the math wizards could help you with, but that is basically it -