in situ/fixatives

[Phillip c. Hartig]

I have been doing in situ hybridization utilizing parafin embbeded
specimens.  We use oligomer probes and paraformaldehyde preservation.  I
have some studies planned which could be scheduled more effeciently if I
can kept the specimens in the paraformaldehyde for periods longer than
overnight.  Most protocols call for processing the specimens at least to a
70% Et-OH step before they can be storred.  My question is this: If I
preserve the tissue in freshly prepared 4% paraformaldehyde/PBS (4 degrees
C), replace it after overnight fixation, how longe can i store the
specimens in the fixative at 4 degrees and not alter the in situ results? 
The tissues will be heavily digested with proteinase K immediately prior
to hybridization and the probes are approx 70 bp in length.
Thank you