Studies have shown that Synthetic Pyrethroid Termiticides such as Bifen usually last 1-5 years when applied outdoors for termite control. The label directions only allow for retreatment of these products when their is evidence of active termites or a chemical barrier breakdown such as plumbing leak, foundation excavation, etc. Here is a link to the label - http://www.epestsupply.com/images/Pro...
There is no standard time for reapplication other than to retreat or reapply whenever the above conditions exist. The option is to install a termite baiting system or to treat with a much longer lasting termiticide such as Termidor (15+ years lifespan).
Hope this helps.