Is it ok to keep reusing a water bottle indefinitely?
The general prescription for water bottles is to recycle them, not reuse them. According to a University of Virginia paper:


  Reuse of polycarbonate plastic bottles is generally not recommended by
  commercial bottled water manufacturers, as it may pose a health risk
  from two perspectives. First, everyday wear and tear from repeated
  washings and reuse can lead to physical breakdown of the plastic, such
  as visible thinning or cracks. Bacteria can harbor in the cracks,
  posing a health risk. Secondly, reuse of plastic water bottles can
  lead to bacterial contamination unless washed regularly. If a consumer
  wishes to reuse a plastic water bottle, it should be washed after each
  use in mild detergent only and rinsed well (9). The plastic should not
  be subjected to extreme, hot temperatures or harsh detergents, and
  should be carefully inspected for physical breakdown prior to reuse.


Additionally, the narrow opening can make it difficult to clean and dry properly.