I suppose you use EDTA. It is slow, even for mouse bone it takes 4-7 days. Regular solution replacement will speed up process, but not drastically. And what tissues are you talking about? Big specimens are nearly impossible to fix properly, and after 4 months without fixation tissues will look not good. May be you can cut slices of specimen from the places of interest? If it is a collagen you are interested in, it is almost non destructible and you can use stronger acids, like 30% phosphoric or 5% nitric.