How long is the typical attention span for sitting and doing completing activity (e.g., reading or writing) of a 5-year old boy?

Depends on the boy: For some 5-10 minutes. For others 30+ minutes when it interests them. Boys tend to be more physically active than girls and tend not to sit still for extended periods of time especially if bored. This question has to be answered in the context of what this child is doing when and where and what is the goal to be accomplished. Is this s a problem at school? Home? Etc.

Huge range of normal: Five year old boys are widely variable as to the "typical attention span". There is such a huge range of normalcy that it is difficult to state an exact number that would be meaningful for everyone. A very crude rule of thumb--which may not be applicable to your son--is about 1 minute per year old; however, there is such variation, so to say that your son should focus for 5 minutes might be in err.