Connie, Microwave fixation can be a boon to the histologist.  There are 
variable that must be considered, but to answer your questions simply:
1)  the size of the tissue is important.  Large pieces of tissue 
will "fix" (denature) on the outside to the point where the tissue can be 
cut more easily (not so soft).  Thin slices are then cut and fixation 
continued.  This is particularly important with large, whole tissues, 
such as brain, liver, etc.
2)  microwave fixation can be started at any point.  Fresh tissue can be 
placed in the solution, microwaved, cut, and then placed in formalin to 
be held for processing; or tissue already in formalin can be microwaved 
to finalize fixation.  There is no set rule.  
3)  there is no specific time for the process to take place.  Needle 
biopsies will fix in 15-30 seconds, others 3-5 minutes, watery tissues 
like brain take longer.  You will have to look at the tissues to be able 
to tell the end point.  This will give you a kind of visual QC to be able 
to gauge future tissues.
4)  finally, the number of tissues that you are fixing at one time will 
also affect the length of time the tissues must be microwaved.  There is 
a lot of "tinkering" to do at first, but once you get the times done you 
will find it a boon to your work.

I hope this helps.  If you have any other questions, you can contact me 
directly but e-mail or phone.  Good luck, Cheryl
Cheryl Crowder, BA, HTL(ASCP)
Chief Technologist
Department of Pathobiological Sciences
School of Veterinary Medicine
Louisiana State University
Baton Rouge, LA  70803

(225) 578-9734
FAX  (225) 578-9720