Generally you will have few commercial brewers that will reuse it more than 5 or 6 times with out re-culturing; and re-scaling up a starter. Ignoring mutation it could theoretically last for ever, but we cannot ignore this.
For ale yeasts you would ideally top crop at high krausen and repitch within 24/48 hours. For a lager yeast you would bottom cop and again reuse quickly. If bottom cropping for ale or lager as is common these days due to tall cylindrical fermenters then taking neither the first or last to flocculate is a must as otherwise you will put selective pressure for early or late floccuation.
Reasons you would not want to reuse ad infinitum are (in no particular order):
If the culture gets contaminated with bacteria or wild yeast it is time to ditch it and start again. Unless you fancy trying an experimental wild batch, but 99.9% of the time ditch it.
- Acceptance of style drift
How crucial is it that you make an identical beer every time? For a brewer making 1600HL batches of Budwieser it is critical, for a craft brewer it may be as important or not; it is a decision for the individual.
The rate of mutation, determines how quickly your yeast will drift form the desired profile. The rate that yeast mutates is affected again by many stress factors: the optimal temperature range for the strain, the strength of the brew higher alcohol level tend to cause more rapid drift, the number of generations more generations leads to more drift, under or over pitching can stress yeast and there by increase the rate of mutation, under aeration of the wort is another factor.
Once you have got to the point where your style drift or contamination is too much you have a few options.
1) Reorder from your stockist, very easy if you use a off the shelf yeast.
2) Reculture from a slant, hopefully you have kept some slants of your yeast in a -80C freezer.
3) Reculture from your last slurry, plate it out, pick a colony that meets your expected morphology.
Finally as Dustin Rasener suggested in the earlier comments get a copy of Yeast (http://amzn.to/iiDdtA), it is an excellent book.