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.