How long to get an accepted share?
The Bitcoin network has a global block difficulty. Valid blocks must
  have a hash below this target. Mining pools also have a pool-specific
  share difficulty setting a lower limit for shares.


ASIC hardware is required for profitable mining because the current difficulty is extremely high as a result of increased hashing power coming online. GPU mining is unable to compete with this specialized hardware. It is most likely the case that your GPU is unable to find any valid shares within the ~10 minute timeframe that occurs between discovery of new blocks.

See this response for more information