How long does lidocaine usually last ?

1/2 to 1 1/2 hours: Depending on where the Lidocaine is placed, the relative blood flow and the ph level of the tissues can affect how long Lidocaine lasts. It may be necessary to mix Lidocaine with other agents such as Epinephrine which help keep the Lidocaine in the tissue. Mixing it with long acting agents such as Marcaine can also lengthen the time that Lidocaine lasts.

It depnds: Duration of local anesthetics depend on many variables, but here is a verbatim note about lidocaine/epi from lexi drugs: "onset of action: peak effect: ~5 minutes duration: ~2 hours; dose and anesthetic procedure dependent".