What level of frequency to retain strength?

Both facts and opinions are welcome. I read that Zatsiorsky recommended training the same muscle group 2 days a week to retain strength. However, this does not say how many days apart these sessions should be. I'm also wondering, due to specificty of motor recruitment patterns, if the statement should be rephrased. Should it be redefined as training the same motor units twice per week?

Well, it's both. Keeping the muscle does require a stimulus (although not as often as 2x a week unless you're on a hypocaloric diet) and the motor patterns do require the 2x a week. Best bet is to keep them 3-4 days apart.

But I don't know why you would want to maintain strength instead of gain it so...

The main reason why I was interested in this was for periodizing routines. I know most people suggest you do not grease the groove with more than two exercises, I guess for both time constraints, and fatigue buildup (if the exercises/muscle groups are closely related).

I have to look at it again, but Zatsiorsky had a graph in his book which showed training and detraining of frequencies of everyday, 2 days a week, and then a hybrid which consisted of everyday followed by a switch to two days per week. Every group continued to make progresss. So, it suggests to me that you can come off a grease the groove style (high frequency) training and still keep the gains until you totally ditch the exercise and allow detraining to occur. Some people were complaining about losing 8-10 pull ups off their max (about 50% or more of their gains went bye-bye) when quitting grease the groove and resuming their original plan, whatever that plan may have been. I've just been thinking about this as a strategy for producing fast gains, reducing boredom, and breaking plateaus.

Ah, now I see what you were going after. Strength after GTG is always a good idea becuase most of the strength is neurological in nature. Doing a strength cycle afterwards at least guarantees some solidification of the strength.