I share your frustration. After many chat sessions, phone calls to tech support, and a stop at my local VZ store, I conclude VZ is keeping the expiration dates a secret. Even the UN-allocated escrow money on a jet-pack account can and will be confiscated if you fail to meet some very obscure rules. VZ has a technically excellent system, but their business policies are just asking for a class action law suit. Something has got to change.
I have the same frustration. The only way to get the expiration date (before which one has to add money to the account) is to call 888-29406804, enter your mobile phone number and follow the prompts. It will tell you the current balance, and the critical date you need to know. You can add credit by phone if you wish.
I know, it is ridiculous. Had to search Google to find the info. The only place on the Verizon website that I have found it is on the page where you select a prepaid service plan. I tried to add a link but the Verizon gods won't let me paste it in. As of today, the refill expiration dates are as follow: