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A good yeast sample can be retrieved, stored & reused regularly across brews. Is there a limit to the number of generations a sample can last?

I've heard that after so many generations a strain can mutate into something different, is there a rule of thumb for how many uses this is? Ignoring mutation, can yeast otherwise last forever if it's properly stored & handled?

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In theory, baring any outside influence in a completely 100% sterile environment, a yeast strain would be good to use over and over again. The issue arises with sterility and outside contamination. I personally don't feel that there is an exact number as to how many times it can be done simply due to the fact that every situation is different and every time you harvest you are going to risk outside contamination.

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This answer from memory only - I hope I have been accurate:

(See comments for strikethrough reasons) Assuming that the yeast that is re-used comes from the conditioned beer not the fermenting bucket, and (like the answer before me @Bullet86) also that there are no contaminants, there is no reason why the strain shouldn't continue to be used.

Commercial brewers invest in their yeast as it is an extremely important aspect of their beer and flavour - it is stored in yeast banks, should they have a disaster they can 'withdraw' a sample.

I have witnessed a bath tub full of live yeast being used in a commercial brewery. I also had the luck to use some of this living yeast and my subsequent brew took on some of the characteristics of the commercial product. Their strain had been used over and over for years as it imparts its own flavour to the finished beer.

Keep it clean and healthy and it will be fine to re-use, but remember to re-use yeast from as late in the process as possible as this will be the best yeast for the job.

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If you always use yeast from as late in the process as possible (i.e. the yeast from the secondary, or the sediment from a bottle-conditioned beer), you're going to be selecting for the least-flocculent yeast, and subsequent beers will take longer and longer to clear. According to Yeast (amzn.to/iiDdtA) by Jamil Zainasheff and Chris White (of White Labs), the best yeast for repitching is the middle layer of yeast in the primary fermentor (neither from the bottom, where the most flocculent, lowest attenuating yeast are, nor the top where the least flocculent yeast are) –  Dustin Rasener May 30 '11 at 16:48
    
Thanks @DustinRasener - Everyday is a learning day. Never believe the first account one reads eh? I am so glad someone commented on this because it made me go and find where I read the 'use yeast from as late in the process' point. It was Graham Wheeler's Home Brewing (1990) and it does state that the yeast found in the bottom of bottles is okay... of good parentage...and has come all the way down the line to producing good beer'. It also states that the first crop is unsuitable, as is the sediment at the bottom of the fermentation vessel. –  iWeasel May 30 '11 at 17:40
    
Additional - Now I feel I need to learn more. I will not argue with Mr White, for I would believe him over Mr Wheeler, and intend to educate myself in the finer points of yeast (okay I have too much time on my hands). What a great subject this brewing is! –  iWeasel May 30 '11 at 17:45
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The actual number of harvests depends on strain too. Some strains (german hefe specially) are known for fast mutations - their ability to produce desired amount of esters degrade over each harvest. Others are more immune, like scottish or irish strains.

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