Covered Call Questions
Hello Sam,
Your questions are excellent, but as far as I know, there are no studies to provide the answers you seek.  Nevertheless,,,
1a) The major benefit of covered call writing is risk reduction - and selling longer-term options provides more protection against loss because the price of the option is higher. 
The major goal of covered call writing is profits - at least for most people (although some will adopt the strategy for its excellent risk profile).  Writing shorter term options provides less safety, but greater annualized profits.
Choosing the option to write is always a trade-off between potential profit and safety.  Thus, when you say 'optimal time period' my guess is: longer-term options if you are concerned with the optimal safety of your portfolio and shoter-term if you are speaking of optimizing profits.
1b) As interest rates rise, call options increase in value.  In theory, longer-term options trade at a level that includes an estimate of higher interest rates in the future.  But, if you expect interest rates to rise, it's better not to sell longer-term calls until they do rise.  If you expect rates to fall, selling longer-term call options now is the way to go.
BUT (this is a HUGE BUT), interest rates don't have a large influence on the price of a call option.  An interest rate change plays a minuscule role - when compared with a change in implied volatility - in the price of an option.
2) Yes.  One simple idea is to place a stop loss order when you open the covered call position.  That mans placing an order to sell the stock.(at the market) whit it hits your target and to have an order in place with your broker to immediately buy the call option (a the market) once the stock is sold. 
Another idea is to do a collar spread, instead of a covered call.  In a collar, you not only write a covered call option, but you also purchase an out of the money put option.  That put effectively gives you a selling price for your long stock.  Not exactly a stop loss, but the result is the same. When opening a collar trade, the idea is to try to get a higher premium when selling the call option than you pay when buying the put option.
The optimal stop loss strategy is to place the stop loss so that it is never triggered.  No one likes to lock in a loss, but to be a successful trader it's mandatory to cut losses at some point.  I doubt it's optimal, but if you are concerned about limiting losses (and you should be), you have to choose your SL point carefully.  If you believe in technical analysis, then use your charts to determine the right price.  If not, then allow yourself to lose a certain amount per trade (and no more than that amount) and set the SL accordingly.  Of course, you will have to estimate the price you will be forced to pay for the option when you sell your stock.
Although I am not a put buyer myself, you may find the collar spread works for it.  It gives you a built in stop loss point.  It definitely limits your potential profit from any trade, BUT the added benefit is that you never get stopped out early.  Opening that put allows you to hold (until expiration) no matter how low the stock goes.  With a stop loss order, there is no chance for recovery once the SL trades are executed.
Mark Wolfinger
Hi Mark
Enjoyed reading your article

Covered Call Writing: A Real World Example
by Mark Wolfinger on 01/24/2005 11:59.

I have few questions:
1) Is there an academic study done to find out - what is an optimal time period for covered call? i.e. Every month vs.3-mon vs. leap? Also, rising interest rate environment vs. lowering interest rate environments?

2) Is there a strategy that couples covered call with stop loss? What is an optimal stop loss to increase probability
of success?

Appreciate your thoughts on that!