What is a good/healthy mysql query execution time?
The question is a bit vague, but I think this answer (a) may provide some useful guidance and (b) is too big to fit in a comment.

Capacity planning a server is a bit more complex than that.  You need to take some basic queuing theory into account.  Your model would describe a server at 100% capacity, which is actually likely to cause performance issues.  In practice you would want to aim for the server running perhaps 25-50% capacity at peak load if you want your site to have consistent response times.

Queuing models state that the average wait time increases hyperbolically as the system approaches saturation. 


A system that is 50% saturated has two requests waiting service on average
A system that is 90% saturated has 10 queued requests on average
A system that is 99% saturated has 100 queued requests on average


You should benchmark your queries under load to see how much resource they actually take up.  Then apply a view of your peak load to see how much capacity you need for the system to remain responsive.