I'm not sure why your seller hasn't sent a default notice and moved on but I would talk to your Broker ASAP.
In cases where all were talking, and the closing attorney had a good feel about it, we gave them a few days. If they are not being forthcoming, I would send them the default notice and relist the house for backup offers.
You need to serve your Seller's best interest.
But, if you have dual agency - I would get your Broker involved for a solution.
The answer will depend on the reason why the buyer is in default and how long it is going to take to fix the issue and whether the sellers are willing to wait for it to happen.
Customary: I'm not aware that there IS a "customary" extension when the buyer fails to close.
You say there's no projected closing date. Again, both parties can agree to extend it. But some agents would advise their sellers to put the house back on the market immediately.