The seller of puts has an obligation to buy the shares at the strike price any time the buyer of the puts decide to exercise them. While early exercise is a possibility it doesn't happen that often...and if it does it doesn't change your risk. While it may change your capital requirements to maintain the position you can sell the shares back out any time your brokerage is open.

The reason it doesn't happen early very often, even if you are far in the money, is that the options still have "extrinsic" value. The put buyer would be sacrificing this value to sell the shares at that price. It would make more sense for them to just sell the puts. To calculate the extrinsic value of an in the money put simply subtract the strike value from the price the stock is trading for...the remaining number is the extrinsic value...or time value. (or simply look at the value of the corresponding call...which, since it is out of the money , has ONLY extrinsic value and will be trading at or near the extrinsic value of the put.) If the options are very liquid and efficiently traded these values will be nearly the same.