hello..in the guideline it is not rigidly mentioned that we have to observed the animals for up to 14 days..!! Sometime observation may be extended more than 14 days and again that is based on the nature of the test chemical used. You can also terminate the study before 14 day if you are able to justify the normal observations of the animals...hope this is suffice 

Acute toxicity studies are performed to monitor the signs of systemic toxicity (like body weight, feed consumption reduction) after single dosing of test item of unknown toxicity. Duration of 14 days (minimum) would give few time points (as weekly or bi-weekly) to observe these parameters and to arrive at a conclusion on possible systemic effects and at times to assess the reversibility.