As stated earlier, implants add to a good management and nutritional program.  That being said, implants are the best ROI management practice available to cattlemen - over a 200 day feeding period, implants will add about 50#s to the finish weight of a steer - so that's $75/head right there.
Feedlots will pay a premium for non-implanted calves - this has more to do with the fact that they get more bang out of their implant buck if the calf has never had an implant, than any sort of demand for all natural calves.
The recommended days on the box are often optimistic, and the actual days depends on the size of the cattle and the energy level of the diet.  Ralgro is a great example - put it in a young calf still on the cow and it's probably going to work for 120-150 days.  Put it into a 5 weight calf on a feedlot diet and it's gone in 45 days.  
When re-implanting, it's best to increase potency.  For cattle sold live, we'll typically start with 90-120 days with an estradiol implant such as Synovex, followed by 90-120 days of a medium dose combination implant such as Revalor IS or Synovex Choice, and finish with 80-90 days of a full dose combination such as Revalor S or Synovex Plus.  The Component line of implants has all of these options with an added dose of Tylan to reduce implant site infection and improve absorption of active ingredients.  For cattle sold on a grid, we may replace the last full dose combination implant with a second medium dose combination.
Edit - These are for calf-feds, for yearlings simply drop the first implant.