The timeframe for a release clause is usually 72 hours. This means when an accepted backup offer comes in, the buyer is required to remove the contingency within 72 hours or the contract is void.
PS means pending sale but continue to show. This is the usual status right after a contract is accepted but the buyer has contingencies to satisfy such as inspections and financing.
PN means pending sale do not show. This is getting closer to closing and the buyer has most likely removed all of their contingencies.
Agents use the rules I have just described as guidelines. Occasionally a home goes directly from active to pending do not show. Occasionally, a home may have a release clause right up until the house closes escrow.
Your agent should consult with the listing agent of the home you are interested in and find out if there will be a possibility of you writing a 'backup' contract. If the listing agent feels the current transaction has a chance of falling apart, you may want to be the first in line without it going back on the market and being open to anyone else who may be interested.
Backup contracts are not common and may prevent you from purchasing the next home that comes up. You'll need a release from the backup before you can buy another home or you could end up buying two properties. A good agent is going to be able to counsel you on all of these issues.
Short answer: Skip the pending do not show properties. They're gone. The pending continue to show properties are probably gone too. Your agent should know how to handle all of this and advise you where to concentrate your efforts in finding a home.
Coldwell Banker Premier
Cupertino and Silicon Valley