The bank does not have to abide by any of the timelines in the contract. However, they give you the option of terminating the contract if you haven't gotten an answer in the time frame. I can tell you that as an experienced short sale agent, the shortest approval time I will write into a contract is 90 days and that would be optimistic.
I've seen sales approved (or counter offered or rejected) within 90 days of a BPO but I've also had them run much longer.
I am so sorry that your Realtor did not explain the short sale process to you up front. It is not a guaranteed purchase. The bank can come back and say no, particularly if the property was sold way below market value. The only person controlling the final yes or no answer is the bank. And they do that on their schedule which is generally 90 days to 9 months from the time you sign your contract.
Best of luck!