If One Silicone Implant is Leaking, How Urgent is Explantation? Would Insurance Cover This Expense? I

I agree that a leaking silicone implant should be replaced sooner rather than later.  Health insurance does not cover the consequences of cosmetic surgery so, unless the implant was inserted during a breast reconstruction, its removal and replacement would not be covered.  If you had surgery in the US with implants purchased from a US manufacturer, the applicable warranty should cover a replacement implant.  If you purchased an extended warranty, additional funds may be available from the manufacturer toward the replacement surgery. 

Ideally, it's never a bad idea to replace the implant(s) sooner rather later. However, having said that there certainly isn't reason for concern or medical issues associated with having a ruptured implant. 

If you're asking if your health insurance will cover implant replacement, no it won't. This is still a cosmetic procedure. All implants have warranties assuming you had them done here in the states. Check with the manufacturer of your implants to see what the warranty entails.