Duration of Maximum Formalin fixing for tissue - Formalin fixing for tissue (Mar/04/2007 )

Goodday to you all

I hope somebody can please assist me.
I need to find good literature on Formalin fixing, with regards to fixing times for tissue and recovery of DNA. I need to understand what is the longest time tissue can be left in 10% formalin solution. I usually allow my tissues to stay for a 12 hr period at -20 and start embedding the day after.I read that at 12 hours DNA is still recoverable and tissue is properly fixed? I however have tissue that been kept in formalin for months.I need to know the effect of this ? Will there be too much crosslinking at this stage? If anyone can point me to literature that explains this, as the literature ive read merely talks about a 12 to 72 hour incubation time peroid.

any help is greatly appreciated

thanks

143