The nerve block meds they use, such as marcaine, stay in my system for how long?

It varies. Marcaine is a local anesthetic (numbing medicine) that can be used for spinal or epidural anesthesia, or injected into an area to numb it. Depending on which technique is used, the numbing effect can last for several hours or more. Sometimes it's used in a continuous infusion to provide ongoing pain relief for 2 or 3 days. It stays in your system for a few hours after injection, then wears off.