This is where the reading comes in...turbomustangs has many write ups with everything a beginner needs to know.
Basically the turbine and compressor sides of the turbo are measured in terms of a a/r....unless its a holset which is cm^2 i believe but thats another story.
Anyway, what mainly controls spool is the turbine a/r which the guy listed above...a .81 will boost around the time when he wants it. Where as on twins, u would want about half that, or for a 351 or stroker, maybe a .96 a/r. THe bigger the number the longer it will take to spool. The smaller the quicker.
But the other thing to remember is that if u get a turbo with a really small a/r in a single application, sure it will probably boost off idle or REALLY low but it will die and choke the motor off by 4-4500 rpms. Thats no fun
So you gotta size it right. Just go to turbomustangs.com and you will see what sizes you should run on which motor sizes etc and the corresponding spool times.